I usually have an aquascape in mind and then try and find a piece of driftwood that meets the mental image of the driftood. Sometimes while looking for that driftwood, I come across another piece that ends up working better for another layout. Of course, I have reasonable mental pictures of the aquascape before spending the money on it. Collecting driftwood where I am is rather sparce, so I usually have to buy if I am looking for something special. Most good driftwood isn't cheap, so make sure you get the one you want!
Here are my two recent driftwood purchases:
Here are my two recent driftwood purchases: