scrub it clean, boil it to kill any weird stuff that might be on/in it, and then soak it in a plastic tub until it sinks. Some people keep boiling it until the water no longer turns yellow/brown from the tannins in the wood, but the wood I collect has been underwater long enough that I don't experience this.
I collect all of my own driftwood and really enjoy the hunt as well as the satisfaction that comes from doing something yourself. I've yet to see wood for sale in stores that is anywhere near as nice as what I find locally.