Define 'large'. A big bit of slate will weigh down all but the largest bits of bogwood that refuse to sink. Continuing to soak as you're doing with a paving slab or similar on top if it's still floating will help in the meantime, but if when you come to put it in the tank it's still floating, a large bit of slate will keep it pinned down until it decides to sink on its own (unless, as I say, it's massive). Beyond that, you're SOL apart from weighing down with something inert and metal (which I can't think of at the moment).