Don't have a clear pic at hand of a spear, but here's a bow with a grain effect:
And a sword (lowest angle shows it best):
This was done with a dark shade of brown, adding a considerable amount of beige for the highlights. For a warmer colour (instead of this grey hue), add yellowish or more orange-brown colours. Add some water to the paint (otherwise, the transition will be very sharp; in addition, there can now occur some diversity in the thickness and thus colour intensity of the layer), get a fine brush (with relatively little paint on it) and go along the length of the weapon. Especially for larger pieces, break up the line, or bend slightly up or down, then start another line underneath/above it. Add parallel lines at about the same distance as the thickness of the line.
Contrast between the layers depends mostly on the desired effect (how prominent?) and the amount of layers you're planning to do. I personally prefer relatively few layers (otherwise it barely shows, it will be more of a gradual transition), and any further highlights should be on the previous lines, or the effect disappears.
Hope it helps a bit.
