To give the aging process (darkening the wood's natural color) a head start,
you can expose the wood to bright sunlight (bottom section of the sample).
After construction/assembly, place the piece near a window that gets
direct sunlight. Rotate the piece 90 degrees daily to even out the effect
on all the sides.
I've used this method a number of times and found that the UV rays from
the sun will darken the wood a fair amount in about a week. After that,
the effect is a lot less dramatic and seems to peak after 3-4 weeks.
If you leave the piece in direct sunlight for too long, it will start to
bleach the color out of the wood (it takes months for a noticable effect).