An often overlooked problem with stealth is movement speed. The answer?
The ever humble Expeditious Retreat spell. The +30' speed bumps up most normal characters to 60', letting you happily stealth along at 30' a round, never slowing down, always unseen.
Grab it however you want, but even just a simple Sorc or Wizard dip will do. Or grab it via the Cleric "Celerity" domain, which comes with a +10 non-typed movement bonus as standard, a big known spell list, and another domain for good measure. 13Wis isn't terribly hard to work into a build, and if it makes you a way faster stealth master, it's worth it.
It also bumps up your Tumble speed by the same margin, making you stabbier by way of flanking and SA (or just making it easier to run in from concealment).
Getting +CL is a great if you can. The Practiced Spellcaster feat (from Complete Divine pg82) is easy, eventually making that caster dip +4CL, turning Expeditious Retreat into a 5min duration spell. That's pretty much an entire encounter, even if it's fiddly and long and stealthy. The extra movement is handy for those invis rounds as well. Sure, you'll need 4 ranks of Spellcraft, but you won't be wanting too much for skill ranks in a stealth build anyway.
So yeah, grab a Cleric (Celerity/whatever) level, the Practiced Spellcaster feat, and have fun. A 40' move Whisper Gnome, with 70' encounter speed and 35' stealth/tumble (once you cast the spell) is awesome. You can jump pretty far too, if that's your fancy. And have a bit of spontaneous healing backup if you really need it (you'll probably never prepare anything other than Ex.Retreat, but you do have the whole cleric list available if you want some utility, or during downtime/RP sessions). Three encounters a day at 70' movement is pretty good for a dip, whatever else it gives you.
Grabbing the Magic domain alongside Celerity is pretty handy in low-magic campaigns. There's all kinds of useful wands and scrolls in these sorts of campaigns, especially for a stealthy person, and cutting down on Use Magic Item rolls is a godsend early on (when you're a bit low on skill ranks to spread around), and can still be useful later (you really don't want to roll a 1-2 at a vital moment for some things). Being able to use all the Cleric and Wizard trash is great, and you may never really get the chance to use anything else. So why not auto-suceed at it?