My recipe isn't exactly that great, but here's what I do:
1) Paint a medium brown of some sort as your main colour. If you want a redder wood or a lighter wood, alter the overall recipe to suit.
2) Choose a direction for the grain.
3) Paint small semi-random lines along the axis of the grain you've chosen in a mix of darker brown, like burnt ochre, and your main colour.
4) Paint small semi-random lines along the axis of the grain you've chosen in unaltered darker brown.
5) Paint small semi-random lines along the axis of the grain you've chosen in a much lighter cream or beige.
6) Use a warm brown ink glaze to unify all the lines with the middle tone.
Example (with a redder, slightly darker base):