I usually place a rock ontop of it. If you want something less noticable you could get those plastic ties (can't think of their name) and attack the driftwood to the rock underneath it.
I remember the name now. Use those zip ties together depending on the size of your driftwood and then attach a piece of slate or a rock. You could use a rubberband too if you have large enough ones.
Get a small piece of acrylic , drill holes in the wood and acrylic and ziptie it to the acylic cover with substrate .. holds it down nicely . you can also use screws to screw it to the acrylic