My lfs said that boiling it in a salt water solution is best to get the tannins out. But my driftwood was too big so I just soaked it in the laundry tub for 2 weeks. Some days I would put in hot water in the tub, but most of the time I just put cold water.
I scrubbed it a lot with a nail brush, I even got a plain toothbrush to scrub it, but there were wood parts that were very soft as I think part of the wood was rotten so I got a stainless steel metal brush from the hardware store and used that on it... worked a charm, took heaps of dead wood off which was causing the water to go very dark and murky.
It's in my tank now and is not leaching any tannins out.