All driftwood in my tanks anymore is stuff I collect myself. There's more than one approach, depending how thorough you want to be, what suits you, the environment the piece came from, etc. (by environment I mean, such as underwater, sitting on the ground, dry and sun bleached, etc.) Some treatments are: heavy salt water solution, bleach solution, slow heat in an oven at 150 or similar, really hot water baths in a rubbermaid type tub, etc. If you bleach it you want to let it air out until bleach smell is gone, if you salt it, you'd want to do a few rinse cycles in a tub.
Personally, I usually power wash externally to get any obvious dirt, mold, etc. off-- unless it's a clean looking piece already-- then I either boil if I can fit into a pan (I've done the boil one side then the other method) or do hot water baths in a bucket or plastic tub. The other factor is how much tannin the piece will leach out, how much it will discolor your water. Boiling releases tannins faster than anything else ime, otherwise (if the piece is too big) repeated soaks in a tub until tannin left in the water is acceptable to me for whatever tank it's going in.