I usually just soak it in a HOT bathtub of water.. hot as in, hot water only, no cold. Just be careful not to get burned! You could even boil it stovetop if you have a pot big enough.
I just got a ton of driftwood from Lake Erie, and I plan on doing just that! It is pretty "cured" naturally, but I still would want to kill off any nasties that may be living in it.