I second that advice! My own take on it, for what it's worth:
I started off with herbalism and skinning and the difference between the early level bank balance of this character and my last one (leatherworking/skinning) is remarkable. With the money you gain through gathering materials - and don't fritter away on levelling your profession - I've found I can generally buy stuff on the AH that's as good as, exactly the same as, or better than, what I could have made.
Once I hit lv35 I used some of my saved money to help power level engineering, which is brilliant for PvP. As a warlock, I think one of your best advantages is the variety of your attack methods - dps, damage over time, pet damage, and crowd control stuff like fear and succubus seduce - so I think anything which gives you more fighting options, like engineering, is a massive bonus. Nice stun effects off the bombs, for example.
I've take alchemy as a second profession. I think that's probably the one with the next biggest PvP value. I really like spending time making cash at the AH though, so I'm usually ok for gold. If you're not into that, I'd definitely recommend mining to back your engineering up.
I hear there's a superb endgame warlock robe that only tailors can make and wear, but I've also read that there's an alternative usable by anyone which is comparable, although not quite as good. That might affect your decision.
I'd also hugely recommend getting first aid as a secondary profession. With your crowd control able to take people out for a good ten seconds or more at a time, but with damage breaking the effect, that's the perfect time to heal yourself. With a healthstone, a healing potion and a bandage, that's three heals on seperate cooldowns you can use, with no mana cost (apart from making the healthstone beforehand).
Cooking too is a good one for us locks, since your can get decent stamnia bonuses through eating, and we do like our stamina.