My lfs here bolts some of theirs down to pieces of slate. However, I bought a piece of African root that wasn't attached, and I soaked it in some used tank water (changed the water during weekly water changes) for about 3 weeks. I know it was sinkable before that (but maybe that's because it's root), but I wanted to get all the tannins out as well.
There were several people posting awhile back about boiling their driftwood (Neilanh, I think, and someone else who locked himself out of the house while the pot was boiling). Anyway, if you haven't already done a search, there are some pretty recent (maybe about 3 months ago) posts about preparing driftwood. And I think there may even be a sticky at the top of the planted tank forum about it.