First aid is pretty much a must have. Because of the warlock's reliance on converting health to mana later on, you'll make good use of being able to bandage yourself. This requires a lot of cloth, so I would suggest away from tailoring.
Herbalism/Alchemy is a decent combo, though warlocks don't particularly need potions more or less than anyone else.
You could try mining/engineering. There are plenty of cool things that would work great with warlocks.
I think the best bet is simply taking two gathering type skills, though. Skinning (because it doesn't require the minimap) and then either herbalism or mining.
If you're other character is a leatherworker, skinning/herbalism is an obvious choice.