Boiling can help speed the saturation process. When the air inside the wood becomes heated it will expand and be forced out of the wood. When the wood cools little vaccums will be created in those pores. If you leave the wood in the water it will pull water into those spaces (if you take it out of the water it will pull air back in).
I siliconed my driftwood to large slate tiles and after a month or two they started to come loose. They're sunk but they move around, which is not what I wanted. It may be a good way to sink the wood if you want that flexibility later.
Next time out I'm going to try epoxy and screws, but this is a very disruptive kind of thing (best for the initial setup, I don't know how practical it would be for an inhabited tank). I have a test tank where I epoxied some wood down and it seems to me like it'll come loose again. The wood gets a little soft as it saturates and it seems like its enough to break the seal under stress. Since I like thin branchy wood that sticks up like roots my wood gets banged around during maintenance -- this may not be a problem with bigger and flatter pieces.
The plan is to epoxy the wood into position and then screw through the bottom of the slate to fix it into place. This should do the trick, but it makes remodeling a little difficult. I went to the Home Depot and bought a few 12" square tiles and some eggcrate (lighting department, the plastic grids that go in flourescent light fixtures). You'd also want some epoxy, some stainless steel screws, and a masonry bit to drill the tiles. (BTW, I found if you glue, epoxy, silicone to the bottom of the ceramic tiles the rougher surface will give you a better grip).
By gluing first I can get everything positioned as I want it, which I think would be much more difficult drilling freehand. The screw heads sitting on the bottom glass and loaded up with everything in the tank could create pressure points, which could create cracks. Cracks are bad. So… cut the eggcrate to fit inside under the whole area involved (I'll do the entire bottom, you could do two tiles and enough eggcrate to fit under them). You can just snap the little pieces of plastic with a pair of needle nose pliers. Slate/wood assembly goes on top of that, gravel and rocks goes on top of that.
Or glue then screw the wood onto a tile and then glue (silicone or epoxy) another tile (or 2 or 3) to the bottom of that. More portable and less need for the eggcrate. You could give it a very substantial base this way, I'd think you could get at least as heavy as you'd be able to with lead.
Lead is poisonous for the fish too. If you decide to go that route I'd drill holes into the thickest parts of the wood, pour the lead in and seal the holes with silicone. But I wouldn't want to risk eventual leaching, so I wouldn't go that way.
Sorry that got so long, but I hope it helps…
