you can also try to boil and soak the driftwood for some time in another container.after some period of time the driftwood will automatically go down and then you can place it in the tank.
you can get a piece of rock or slate and use sumo or gorilla glue to anchor it. just be sure to let it fully cure (i would wait a few days JUST to be safe).
What i plan to do for my big tank is lay egg crate down before the substrate and tie the floating driftwood to the egg crate. Then place the substrate and rocks on top of the egg crate.
Granted you'd have to really get in there to put it in with the sand already in there, but that wouldn't be noticeable, and it would sink it for sure.
i drill holes in stone pieces like slate, or soapstone. if you buy driftwood from the lfs it is attached to slate using 4''-6'' screws. you can get away with 2 or 3 screws per piece. i've even just used one before! drill from the bottom of the stone and you dont see a thing.