Boiling works to take the tannin out, but it doesn't work for everything. If your wood is particularly thick, boiling the crap out of it for days may not do the trick. If thats the case, you'll just have to stick it into your tank and live with the brown color for awhile. If your piece of wood is too large to fit in a pot, then soaking it in a bucket of water for a few weeks (with frequent clean and hot water changes) also helps.
Just FYI, tannin doesn't harm your tank, it just makes it unsightly (to a few ppl, not to others). Tannin does drop your pH slightly (by softening your water) but that shouldn't be a problem for most people.
HTH
-Richer