If you get the Malaysian driftwood, it will soak up water and sink. It takes a couple of weeks. Otherwise see above. Malaysian is really dark, almost red. Regular is lighter in color and will never sink.
if the druftwood is shaped properly you can put heavier objects such as rocks and or decors on top of it until the wood has become completly saturated.
Its also a good idea to attack it some way as suggested above to another heavier object and make sure all contents are marine safe and compatable
good luck :
I've a piece of slate that's 21lbs to hold the driftwood down in my tank.
I drilled two holes through the slate and countersunk them slightly.
I used two stainless screws to secure the slate to the wood.
one thing u might wanna note is, once the wood gets saturated with water, it will sink on its own. Alot of the time, it will work if u just let the wood float on the surface and slowly saturate... most ppl say it only takes a few days. But weighing it down doesnt hurt either, of course
I used a old under ground filtr plastic piece i had laying around and screwed it to the bottom of the wood,threw all the substate and a coulple larger river rocks on top of the UGF peice.After about 3 weeks it was saturated enough to where I was able to remove the river rocks.