Glue it to a piece of slate or bury it under rocks. You can try boiling it or letting it sit in water for some time to see if it'll sink on its own. But from my experience; if it doesn't sink when you buy it, you shouldn't expect it to after you try to treat it.
It depends on what type of wood it is. Weighting the wood underneath rocks or something in a tub can do the trick. I would put a powerhead in with it to keep the water circulating and prevent stagnation. Frequent water changes help too. Using warmer or even hot water helps too.
I have used a large rubbermaid tub, Fill with hot water and place something heavy on top of wood to hold the wood down. With pieces of wood that big it will take some time for them to become waterlogged.