Mage-Thief (Without losing any caster levels!)
From D&D Wiki
This character build is for those of you who want to combine some of the elements of the rogue class and maintain a full-leveled spellcaster. Through the use of levels in: Wizard(PHB), Abjurant Champion (Complete Mage), Unseen Seer (Complete Mage), and Arcane Trickster (DMG), your player gets access to sneak attack, some Rogue skills, and increased attack bonuses all while keeping your character level and caster level equal!
Dungeon Master's Guide,
 Game Rule Components
Player's Handboook: Wizard Levels, Feats, Skills, Etc.
Monster Manual: Gray Elf Race
Dungeon Master's Guide: Arcane Trickster Levels
Complete Mage: Abjurant Champion Levels, Unseen Seer Levels
 Spells, Powers, Soulmelds, Stances, etc...
At least one abjuration spell (for Abjurant Champion) At least two divination spells (for Unseen Seer)
Starting Ability Scores (Before Racial Adjustments): From Highest to Lowest in priority: Int, Dex, Con, Wis, Str, Cha. For example, a typical starting character would have: Str: 10, Dex: 14, Con: 12, Int: 16, Wis: 12, Cha: 8
Race (Templates): Gray Elf
Starting Racial Traits: As Elf, but abilities are adjusted as follows: Str -2, Con -2, Dex +2, Int +2
|1st||Wizard||+0||+0||+0||+2||Combat Casting||Scribe Scroll, Summon Familiar, Spellcasting||Elven Proficiencies (Longsword, Rapier, Longbow, Composite Longbow)|
|4th||Wizard||+2||+1||+1||+4||Ability Increase (Int)|
|5th||Wizard||+2||+1||+1||+4||Bonus Feat: Spell Focus(Evocation)|
|8th||Wizard||+4||+2||+2||+6||Ability Increase (Int)|
|10th||Wizard||+5||+3||+3||+7||Bonus Feat: Greater Spell Focus(Evocation)|
|11th||Abjurant Champion||+6||+3||+3||+9||Abjurant Armor, Extended Abjuration, +1 Level of Existing Arcane Spellcasting Class|
|12th||Abjurant Champion||+7||+3||+3||+10||Greater Spell Penetration||Swift Abjuration,+1 Level of Existing Arcane Spellcasting Class||Ability Increase (Int)|
|13th||Abjurant Champion||+8||+4||+4||+10||+1 Level of Existing Arcane Spellcasting Class|
|14th||Unseen Seer||+8||+4||+4||+12||Sneak Attack Damage +1d6, +1 Level of Existing Arcane Spellcasting Class|
|15th||Unseen Seer||+9||+4||+4||+13||Quicken Spell||Advanced Learning, Silent Spell, +1 Level of Existing Arcane Spellcasting Class|
|16th||Unseen Seer||+10||+5||+5||+13||Divination Spell Power +1, +1 Level of Existing Arcane Spellcasting Class||Ability Increase (Int)|
|17th||Unseen Seer||+11||+5||+5||+14||Sneak Attack Damage Bonus +2d6, +1 Level of Existing Arcane Spellcasting Class|
|18th||Arcane Trickster||+11||+5||+7||+16||Alacritous Cogitation(Complete Mage)||Ranged Legerdemain 1/day, +1 Level of Existing Arcane Spellcasting Class|
|19th||Arcane Trickster||+12||+5||+8||+17||Sneak Attack +1d6, +1 Level of Existing Arcane Spellcasting Class|
|20th||Arcane Trickster||+12||+6||+8||+17||Impromptu Sneak Attack 1/day, +1 Level of Existing Arcane Spellcasting Class||Ability Increase (Int)|
|21st||Arcane Trickster||+12||+6||+8||+17||Epic Spellcasting||Sneak Attack +2d6, +1 Level of Existing Arcane Spellcasting Class||Epic Attack Bonus (+1)|
|22nd||Arcane Trickster||+12||+6||+8||+17||Ranged Legerdemain 2/day,+1 Level of Existing Arcane Spellcasting Class||Epic Save Bonus (+1)|
|23rd||Arcane Trickster||+12||+6||+8||+17||Sneak Attack +3d6, +1 Level of Existing Arcane Spellcasting Class||Epic Attack Bonus (+2)|
|24th||Arcane Trickster||+12||+6||+8||+17||Automatic Quicken Spell||Impromptu Sneak Attack/2 day, +1 Level of Existing Arcane Spellcasting Class||Ability Increase (Int), Epic Save Bonus (+2)|
|25th||Arcane Trickster||+12||+6||+8||+17||Sneak Attack +4d6, +1 Level of Existing Arcane Spellcasting Class||Epic Attack Bonus (+3)|
|26th||Arcane Trickster||+12||+6||+8||+17||Ranged Legerdemain 3/day, +1 Level of Existing Arcane Spellcasting Class||Epic Save Bonus (+3)|
|27th||Arcane Trickster||+12||+6||+8||+17||Automatic Quicken Spell||Sneak Attack +5d6, +1 Level of Existing Arcane Spellcasting Class||Epic Attack Bonus (+4)|
|28th||Abjurant Champion||+12||+6||+8||+17||Arcane Boost, +1 Level of Existing Arcane Spellcasting Class||Ability Increase (Int), Epic Save Bonus (+4)|
|29th||Unseen Seer||+12||+6||+8||+17||Advanced Learning, Guarded Mind, +1 Level of Existing Arcane Spellcasting Class||Epic Attack Bonus (+5)|
|30th||Unseen Seer||+12||+6||+8||+17||Automatic Quicken Spell||Divination Spell Power +2, +1 Level of Existing Arcane Spellcasting Class|
 Other Components
The most difficult part of this class is the necessity to throw a bunch of skill points into cross-class skills. For Unseen Seer you'll need to max rank the following skills until level 13: Hide, Search, Spot (8 ranks each). You'll also need 4 ranks in Sense Motive and Spellcraft (the latter of which you'd want maxed out as a Wizard anyway). Once you take your first level as an Unseen Seer, you need to temporarily dump all of your extra skill points (6+Int Modifier for these 4 levels) into: Decipher Script(7 ranks), Disable Device (7 Ranks), Escape Artist (7 Ranks). You'll also need Knowledge(Arcana) to be at 4 ranks, but let's hope you've been putting as much as possible. While leveling, try not to spend any skill points on any Cross-Class Skills other than the ones listed above. Any points that you save can be instantly put into practically anything that you could want in your first level of Arcane Trickster!
The reason that this build allows you to take levels in Arcane Trickster is because of your levels as an Unseen Seer, which grants you a sneak attack without having to take any levels in Rogue. There are few levels of Abjurant Champion thrown in for four reasons:
1)d10 hit die
2) No loss of caster level
3)+1 base attack bonus/level
4) Really awesome bonuses to your abjuration spells
The Abjurant Champion class is generally designed for melee/spellcaster combinations, but I personally can't see any reason why every elven mage wouldn't take at least a couple levels in this prestige class. The only prerequisites are: BAB +5, Combat Casting(Feat), Spellcasting(level 1 spells and one abjuration spell), and (Special) Must be proficient with at least one martial weapon. Elves just happen to be proficient with a couple martial weapons, which makes a patient mage a perfect candidate for this awesome prestige class!
If you just can't imagine playing an elf, or would prefer not to take levels in Abjurant Champion, you could easily substitute the levels with either Wizard levels or several levels of Loremaster(you would need to replace Combat Casting with Skill Focus, and move Quicken Spell up a few levels). This would also give you some bonus skill ranks to invest if necessary.
There is a small chance that your DM might try to make a fuss over the sneak attack damage granted by the prestige class Unseen Seer. Just stick to your guns and state that the class deserves the bonus due to all of your invested cross-class skill ranks!
NOTE: If your DM is paying attention he may notice that Unseen Seer doesn't actually grant the Sneak Attack ability, only a bonus to it's damage.... If this is a problem, try taking Hunter's Eye at US2 and getting it made permanent. While this still does not technically let you qualify for ArcTrik, some DMs will allow it as it is now a permanent ability.