1) White undercoat
2) Streak a dark brown like GW Scorched Brown from the ends of the wood inwards, not too far mind you.
3) Streak a lighter brown, such as GW Snakebite Leather towards the centre of the wood. Somewhat overlapping the darker brown and streaking onto the unpainted white.
4) An even lighter brown, like Bubonic or Vomit Brown. Streak it over the previous layers and covering the unpainted white.
The main idea is to keep these colours very thin, so that the underlying layers almost show through. This makes a decent wood grain effect over large wooden areas.
Anyhow, that\'s a quick and dirty method that I\'ve used. It\'s not going to be as good as Margo\'s extremely brave and daunting method of freehanding the wood grain.