lol, this topic comes up at least once a week...
you have several options...
1) purchase some slate (Home Depot has 3/4 inch stuff pretty cheap) drill a small diameter hole in the middle, then screw the piece of driftwood to it... and valla...
2) use fishing line to tie off the wood and attach it to rocks or suction cups in the tank.
3) If you boil the wood enough it will eventually get saturated and sink (theoretically)
I'm sure there's a lot of other ways but those are the approaches I have used successfully. More options will come...