Multi-classingGeneral Disclaimer: An optimization rule of thumb is that caster levels are superior to other abilities. Caster levels with extra abilities are superior to even that. Basically, if at all possible, don't lose caster levels. If you must lose caster levels, never lose more than 3 in a 20th level build, and make sure the benefits greatly outweigh what you are losing.
Clerics are unique in that you get all of your class abilities at the outset, so Prestige classing and Dipping into other classes is much more viable than in some other builds. That being said, the disclaimer about Caster Levels stands. Another important thing to note is that if you plan on using turning for more than just Divine Metamagic, make sure your prestige classes advance turning much like cleric does. Not all of them do, and it is important to note the ones that do for this purpose.
The Soulknife-Soulbow Dip The standard idea with this build is to grab two levels of Soulknife for the throw mind blade class feature and then dip one level of Soulbow. The idea is that Soulbow allows you to apply your Wisdom modifier as damage to the Mind arrows you fire. When in a null psionics field, you can attempt to use your mind arrows regardless. In a psionic-magic transparency game, this means you can manifest the mind arrow in an antimagic field. This dip, while it may seem like it is okay for ranged clerics, is fairly bad. If your DM reads the mind arrow ability to mean that you can make +2 mind arrows at character level 7, you're good, but most likely he'll read it as it was meant to and require multiple levels of soulbow for this purpose. You are better off getting Raptor Arrows and good bow. Still, for a psionic flavor, there are many more worse options.
The Monk-Sacred Fist Dip The goal of this build is a monk/cleric "divine gish." By taking one level of monk (two if you can't spare the feats needed), you then take cleric and Sacred Fist. This prestige class is viable up to two points, depending on your DM. If your DM rules in favor of text over table, take all 10 levels, because it is full casting. If he or she rules the table to be the authority, dip out at level 6. Either way, you'll want a Monk's Belt and something like either Bones of Li-Peng (WoL) or Gauntlets of the Talon (MIC). Skydragonknight has a good example of this combination in the sample builds below.
The Druid Variant Dip Unearthed Arcana has a druid variant that offers a monk's AC bonus, the Track feat, and one Favored Enemy as a one level dip. For certain cleric builds, this could be a good choice. The Monk's AC bonus is always a nice boost, and a +2 favored enemy can be really useful for not too heavy of an investment. Probably the biggest boon of this dip is access to the druid's spell list. You can now activate spell completion items of druid only spells like Owl's Insight
. Now if only there were divine runestaffs...
The Classic Fighter Dip A pretty straight forward dip, one or two levels of fighter. Two levels can get you the Dungeoncrasher ACF, but it also costs you a second caster level. Depending on where you go with this, you can enter Ordained Champion fairly easily (take only 1 fighter level if any) or Bone Knight (you can get away with two here). For the melee focused cleric, this dip is a decent, if archaic, choice.
The Crusader-Lockdown Dip Taking Crusader 1 costs you a caster level but nets you the prerequisites for taking Martial Stance (Thicket of Blades) as a feat. If you want to mix all the wonders of a lockdown build with the power of a cleric, this is a decent method. You could accomplish this with a couple feats, but if those are limited in your design, this is a safe alternative. Another good pro for some people is that the pre-written flavor fits smoothly, too.
A Rogue Dip? Here our goal is multifaceted. Able learner combined with a level of rogue forever gives us the stealth based skills we need without resorting to our domain choices. The 1d6 sneak attack is a nice little boost, too. It's not the clear winner, but it opens up spells like grave strike as having a bit of potential. The kick in power to your Reflex save also helps the one bad save you have. Primarily for skillmonkey clerics, this dip can be worth it.
Dragonfire Adept Dip What the hell does a dragonfire adept have to do with cleric casting? Everything. No, not really. But I love the class, and it does make a surprisingly good dip. You gain a breath weapon, which, while minimal in damage can be boosted with the ever amazing Entangling Breath feat. Also, the Dragontouched feat gives you the dragonblood subtype, which lets you grab some cool feats if that's your goal. The kick to your saves is like icing on the cake. The real winner, here, though, is that one single invocation.
Draconic Knowledge is perfect for Knowledge Devotion builds. +6 unnamed bonus to all Knowledge skills and Spellcraft, plus you can make checks even without ranks. That's useful for both regular and cloistered clerics.
Prestige Class OptionsBone Knight5N As long as you are not barred from using Eberron books, this is one of the best cleric prestige classes around. It is especially ideal for melee oriented clerics. It is a 9/10 Casting class, meaning you only lose 1 caster level. It meshes well with Ordained Champion (see below). The skills are a bit tough to come across, since craft and ride are not traditionally of use to a cleric, but they are worth it for Bone Knights. You are forced to Rebuke Undead for this class, but it is very much worth it. The class stacks with your cleric levels for turning level. On top of all the casting, you gain a boatload of immunities that will make you a beast on the battlefield.
Church InquisitorCD Church inquisitor is one of the very few prestige classes that can be entered before having 5 base classes. Once you have 3 cleric levels, if you are non-evil and lawful, this prestige class is a good choice. You gain an extra domain (Inquisition). The domain can be kept for a nice ability or traded for the Knowledge Devotion feat. The mental immunities and auto saves against certain spells and effects are sweet. The class is good for the first 5 levels. If you are worried about possession, level 8 make a second stopping point. The last two levels of the class really just aren't worth it.
ContemplativeCD You can't take this class until level 11, but it is well worth the wait. At first level, you gain an extra domain and immunity to disease. That's beautiful. Really, many cleric builds just take this one level. If you are looking for more, then the second drop point is level six. At level 5, gaining immunity to poison is okay, but you get the second domain at level 6.
Divine OracleCD This is another full casting PrC. It has either 2, 6, or 10 levels. The Oracle domain is a fairly decent domain, but the second level is where things get nice. You gain an ability that functions as EVASION, but in ANY armor. Heavy armor clerics with evasion is quite sickening. Level six makes you unable to be flanked with Improved Uncanny Dodge. Level 10 is truly the cream of the crop, though. Remember that whole thing about initiative? Well you always get to act in a surprise round, if there is one, so no one can get the drop on you. It take a while to get, but that reward is pretty sweet.
MalconvokerCS An 8/9 casting PrC focused on battlefield control via summons. While it can be argued that wizards or druid do this class better, clerics make damned good Malconvokers. Add some Thaumaturgist on top of it and you're golden. Treantmonklvl20 has a wonderful guide to Mastering the Malconvoker.
Ordained ChampionCC 3/5 casting means you can only lose one other casting level. Another reason I suggest that this build be used in conjunction with the Bone Knight (see above). This class also advances turning. Yay! You gain a third (or fourth or fifth) domain upon entering this. If you don't have the war domain, you'll gain it instead. This class basically forces Domain Spontaneity (War) on you as an ACF, but it is well worth it. The abilities of this class are good for melee combatants.
Prestige PaladinUA This class only exists for a one level dip. If it wasn't actually a prestige class, it would be up in the dip section. The important aspect of this class is that it puts all the paladin spells onto your cleric spell list. In combination with Ordained Champion, A build utilizing this dip can be a powerhouse.
Radiant ServantCD The worst aspect of this class is the Sun domain requirement, but it fits and it actually makes it useful. Extra Greater Turning is amazing. You could dip this PrC for 1 level or stick it through to ten. Level five is probably the best drop point as it nets you a bonus domain. For a Neutral Good cleric, this domain is about as good as it gets. I mean the class grants you weapon proficiencies. That alone is sweet.
Ruby Knight VindicatorToB Remember that crusader dip I mentioned up above? Here's how you make the most of it. This build is often nicknamed the Ruby Knight "Win"dicator. Let me explain why. Full BAB and 8/10 casting. You gain maneuvers and stances (not many, but they're useful in combat... especially since you don't have to spend a feat on Thicket of Blades now). The "win" though? That's the 7th level ability, Divine Impetus. A Windicator breaks the action economy. As many turn attempts as you have, you can spend to gain swift actions. Travel Devotion lets you move your speed as a swift action. Battle Blessing lets you cast Paladin Spells as swift actions. Quickened spells. I think I've said enough.
Sacred ExorcistCD The best thing about this class is that it stacks with cleric for turn undead. It also has some useful abilities for undead or evil outsider heavy campaigns. Full casting progression makes it a viable choice for certain prestige class hungry builds.
Sacred FistCD The text has to triumph over the table for this prestige class to work. By the rules, the text does, but if your DM is a douche, you may not be so lucky. You'll want to take at least one Monk level, like I mentioned above, before taking this class, as it will fulfill the many feat requirements. Stacking monk unarmed damage with full cleric casting makes for a powerful build. See the sample build section for just how powerful.
Seeker of the Misty IsleCD Another issue of text over table. This class is also full casting by the table. Remember to point that out to your DM. The race requirement is harsh, but if you ignored my advice and are playing an elf, the class gives you two solid domains. This PrC meshes well with Cloistered Clerics for the whole skill-monkey cleric angle.
Sovereign SpeakerFoE Another one of those Eberron Cleric PrCs that works wonders. The required feat actually isn't a terrible one. It can be useful for clerics, making you not dependent on your holy symbol. Even beyond the full casting this class gives, is the ability. You gain a new domain at every level. Granted you are limited to not taking too many domains of a certain deity, but you can get some of the best domains this way. Feel free to stop taking the class before you have to start taking any less optimal domains.
ThaumaturgistDMG Your HD in this class is pretty crappy, but the ability boosts of this 5 level PrC are fairly nice. It really meshes well with Malconvoker and makes for a wicked Planar Binder.