The techniques I use depend heavily on the casting. If the wood grain is done in good relief, I use a dark brown undercoat, a yellowish drybrush, and finish with a medium brown wash. If the grain isn't really represented in the casting, drybrushes and washes don't work very well. You can create a grain effect on the surface by using an old chisel-shaped brush, poking it into the paint to get a "spiky" effect, and then painting a single stroke very lightly. I would use a lighter undercoat in that case, with a darker topcoat.