How can I permanently increase my Dexterity ability score?

That's it.

Other than starting from an 18 point base, that's it. In the 3.5e PHB and DMG, that's the maximum.

In Non-Core, however.

The Sources of Bonuses to Ability Scores thread at Brilliant Gameologist's Min/Max Boards provides all known bonuses to ability scores.

In SRD only

The SRD (System Reference Document) is the freely released freely available D&D content outside of web articles and the like. It's available in handy hypertext format at d20srd.org.

Taking a quick look through that, the Whirling Frenzy custom barbarian Rage variant gives +4 dex, the Epic Level rules have a feat that gives +dex, and the monsters included include lots of +dex races that also have a listed LA - and so are playable, if terrible.

With the MM

With the MM1, and the polymorph or shapechange spells, you can boost your base dex quite high, and then add the normal buffs on top of that to have a quite high dex.

It's 'permanent' because those spells can, with Extend and repeated castings, last most of the day.