Welcome to the "I'm waiting for the drift wood to sink club", Crash. It's a pretty big club. You pretty much have two options. Weigh it down, or wait. I'm currently got several large rocks sitting on top a lovely bit of drift wood soaking in 4 foot custom tank. They'd been soaking for a few months and one of the two pieces was still floating. Another thing I have done is to screw on to the bottom of some of my drift wood some Lead fishing weights. Some people don't like that idea of putting metallic Lead into their tanks, but I have pretty neutral PH water and am unlikely to have Lead go into the water. Very acidic or high PH water might react with the metal and if you're not comfortable with that, don't do it. Fish keeping should be about making it less stressful, not more.
Another trick I have done and also seen others do is to screw large plastic pot saucers on to the bottom of the wood. Then you cover the plastic with substrate and that will keep it from floating away. Really large bits of wood may not work like that until they're already half water logged though.
Good luck.