Sorcerer/Warlock multiclass

[Arakune]

I want to have my warlock to eventually have some old fashioned spellcasting. How can I give it to him without losing too many caster levels in the process?

[Snowbluff]

Eldritch Theurge (Complete Mage). Take one level of a caster (Sorc, Beguiler are my faves for this). If you chose Sorc, take Sanctum spell with the one MM prereq. If you chose Beguiler, take Versatile Spell Caster. This will let you meet the 2nd level caster requirement. A less cheesy option would be to use Precocious Apprentice.

If you go Sorc, I'd suggest grabbing some good touch and area (Wings of Flurry, hands down) spells.

On a side note, a Precocious Apprentice Warlock meets both requirements for Eldritch Theurge, and both spell casting progressions, but i specifically says warlock cannot benefit from both progressions. What you could do is dual-progress 2 Invocation Classes, if your DM would allow it. XD

[Jasdoif]

Since Eldritch Theurge has already been mentioned....If you're not sold on Sorcerer or arcane spellcasting, you can qualify for Eldritch Disciple with Cleric 4/Warlock 1. Eldritch Disciple progresses your divine casting at all levels past 1st, so you'd only be two levels behind a full cleric. Cleric 3/Warlock 2 is also valid, if you want more invocations or need to avoid a multiclass XP penalty.

If you can get your DM to accept the adaption listed in the book of not requiring turn/rebuke undead, you could enter with many other divine casting classes (ideally Cha-based for synergy with invocations).