I've actually played a default, 3.5 warlock from 1 to 20, and, just like any other class, you just have to play to the strengths. Warlocks are a one trick pony, so make sure it's a damn good trick and take advantage of the fact that you can switch out invocations when you level (get rid of one that's proven useless). When I got to high levels, we spent most of our time in the underdark, so I dropped eldritch spear, since my warlock darkvision only went out to 60 ft. However, I could defeat even magical darkness to 30 ft. with another invocation, so I took the one that creates a globe of magical darkness. Then, even on the surface, I was awesome. There's also the acid one that negates SR and only pays attention to acid resistance.

Well, I seem to be rambling a bit, but here's the short of it: When all you've got is a hammer, every problem looks like a nail, so when you get one of your precious few invocations, take one that's as flexible as possible, then try to pigeonhole every situation into be a 'nail.'

Also, get scribe scroll and start making all kinds of neat crap to use when you just can't solve the problem with your hammer.