Iterative probability and multi layer defenses.

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Iterative probability is important in D&D. Even if you only have a  1/20(5%) chance to fail that fort save against Disintigrate eventually you will fail and get turned to dust unless you have some other defense.

Lets say you have that 1/20 chance of failing your save, probability is worked out by multiplying together individual cases. For example, the second disintigrate still has a 1/20 change of killing you but the total chance of failing one of those is actually around 90%.
You work this out this way:
First you work out the chance of failure:
1/20 = 0.05
then invert that so you have the chance of success
19/20= 0.95
Then multiply the chance of success together to get the chance at succeeding both saves:
0.95*0.95= 0.9025
Which is around 90% so a 10% chance that one or the other killed you. It also works for more repetitions:
0.95*0.95*0.95*0.95 = 0.81450625
80% or so means thats a 20% chance of death.

The problem in D&D terms is that as an adventurer you will be targeted hundreds if not thousands of times over your level 1 to 20 career and the probability of you failing one save is enormous even with the highest possible bonuses to your saves, however there are solutions.
First is getting your saves to that magical 1/20 chance. This can be achieved with magic items or spells usually but high base saves and good Dex/Con/Wis help too.
Class specific ways of boosting Saves:

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Several classes have ways of adding an additional stat to one of your saves. Best know is probably the Paladin's Divine Grace but other versions do appear here and there, see X stat to Y bonus for more on that.

Other than that, every class has base saves. Certain classes like monk even have perfect saves which certainly help your survival, a good way to make use of this is by multiclassing.
Multiclassed characters saves include the additional 2 points at level 1. Theoretically you could take the first level in 20 different classes and end up with a high set of base saves but this is rather unlikley, still it is an incentive to prestige class and multi class whenever you can.

Next there are items that boost saves, these are generally a good idea to buy no matter what character you have. So far I have only gone through Core so this is nothing like an exaustive list:

Cloaks of Resistance offer a +1 to +5 bonus to your saves and I would recommend this as one of the first items on any characters shopping list. Cost: 1k-25k
Luckstones give you a nice +1 luck bonus to a lot of things including saves for 20k
The Green Ioun stone gives you a +1 competence bonus on several things for 30k

After that are Feats, Iron Will for +2 to Will, Lightning Reflexes for +2 to Ref and Great fortitude for +2 Fort are well known Core options. I would not recommend any of these as anything other than prerequisite feats for prestige classes but if you really need to boost a low save they are there as an option. 

There are also Manuvers in the Tome of Battle that allow you to replace your save with a Concentration check which is significantly easier to boost in nearly all cases.

Finally there are a lot of spells, generally on the Cleric list that boost saves. I will compile a list later but for now remember the rules on how bonuses of the same type interact (No Cloak of Resistance with a Resistance spell for example)

Rerolls certainly help because they make the chance of a failed save much lower. The same 1/20 failure chance results in around a 0.25% chance of failure if you have a reroll rather than 5% which is an enormous increase in survivability.

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One of the best ways to get a reroll is the Luck Domain granted ability that gets you a 1/day reroll on any roll after you make it. This is awesome but unfortunately limited to clerics and the PRCs that grant domains.

Pride domain does even better, allowing you to reroll any roll of a 1 on a save. If you get that magic 1/20 chance on your saves this more than doubles your survivability.

There are also the Luck feats from Complete Scoundrel. Of note there is:
Dumb Luck: spend a luck reroll to treat a natural 1 as a natural 20. and
Survivors Luck: Reroll a failed Save
Both are good if you are interested in boosting your chance of survival.

There are also abilities that allow you to ignore that 1/20 chance of failure once you get your save high enough.

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Steadfast Determination in the PHB2 gets your Con mod to will saves instead of Wis mod which is nice on its own but it also gets the awesome ability to ignore the a natural 1 automatic failure rule on a fortitude saves which is even better.

The Pride Domain is mentioned above in rerolls but it works here too.

Next there are the abilities that make your saves more effective, Evasion and its rarer cousin Mettle are their general names and they really help survival some of the time. Both can be gained through class levels but Evasion can also be had from items.

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Evasion


Mettle
Pious Templar (CD 50) 1st level
Hexblade (CW 5) 3rd level
Crusader (ToB 8) 13th level
Vigilante (CAd 85) 9th level
Witch Slayer (ToM 67) 2nd level
Goliath Rogue substitution level(RoS 152): 2nd, mettle on Fort save only, replaces evasion
Tabard of Valor (CCh 142) Item, 16,000 Grants Mettle when at or lower than half hit points, or Improved Mettle if the character already has mettle from a class or other effect.

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The second section of this guide focuses on AC.
This is generally not a good defense for several reasons but mostly it is because attack bonuses scale with level and magic items when AC generally only scales with items.

Touch AC can however be boosted to a decent level and even your normal AC has its uses at lower levels.

First general AC bonuses, these are not all that useful past a certain level (5-8) unless you want to spend the vast majority of your gold on it because while it is rather hard to push your AC past about 28 a monsters (or Players) attack bonus will keep going up rapidly. Many mid to high level monsters have attack bonuses of more than 35 or 40 so the chances are they are going to be hitting you. What having a High AC does do however is reduce the amount that they can Power Attack for while still being certain of a hit and this in turn reduces the amount of damage you take by a bit.
AC bonuses:

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Armor, generally the cheapest way of boosting AC armor can help significantly at low levels.
Full plate gets you the best AC in core but you need to also take into account the max Dex, here is a table with the maximum AC achievable with each armor

Armor Name	Max Dex	AC bonus	Theoretical MAX AC
Light Armor
Padded	8	1	9
Leather	6	2	8
Studded Leather	5	3	8
Chain Shirt	4	4	8
Medium Armor
Hide	4	3	7
Scale Mail	3	4	7
Chainmail	2	5	7
Breastplate	3	5	8
Heavy Armor
Splint Mail	0	6	6
Banded Mail	1	6	7
Half-Plate	0	7	7
Full Plate	1	8	9

Obviously not every character can hit the Dex bonus required to use padded armor properly (You would need 26 Dex) and not all characters have heavy armor proficiency so it is not quite as cut and dry. In addition to that heavy armor is generally considered suboptimal for various reasons like the speed penalty. 

Touch AC bonuses:

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As above AC is not really a good defence against melee attacks past about level 10 (Even that is pushing it) but there is a way to stop that dragon from smashing into the floor and that defense is miss chance. Miss chances are however a lot easier to come by for mages so this is one of the reasons melee suffer so much in this game.
Miss chances are generally gained through spells and concealment

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Blur: is a good low level defensive spell, its 20% miss chance is generated by Concealment so you also have the nifty bonus of being immune to sneak attacks.
Blink: Provides an even stronger 50% miss chance against most things but magic weapons or creatures that have see invisible or Truesight. Drops to 0% if they have both so it is best used to defend against things like the Tarrasque which have neither and don't have the inteligence to compensate (like a dragon might)

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The next Section of the guide focuses on Critical Hits.

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Immunities are a strong defence, they are sometimes harder to get but being completely immune to an effect is awesome.

Fire and Cold subtypes grant immunity to their related damage at the cost of a 50% increase in the other. It is possible to get both subtypes which makes you immune to both. Then there are creatures that are immune to damage types without the subtype like demons.

Fire and Cold

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Lots of templates and higher ECL races grant one or the other, so many so that they could fill a handbook by themselves but I will note the best ones for players here.

There are also spells and Items that can help in this regard

Other Elemental immunities

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While not generally useful for players demons are immune to Electricity where as Devils are immune to Fire.

Affliction immunities-
Stun
Paralyze
Sleep
Disease
Death Effects
Ability Drain
Energy Drain
Ability Damage
Mind affecting
Poison

In addition if you have a constitution score of - you are immune to any effect that requires a Fort save and does not affect objects.