Originally Posted by JTMSP

"Runway numbers are primarily chosen based on the magnetic heading of the runway. Since a take-off or landing can normally be accomplished on the same piece of pavement in either direction, runways are designated with two numbers, and the number used approximates the direction of travel along the pavement from one threshold to the other. We drop the last number of the heading, rounding up or down as required. If a runway's heading is 020° degrees magnetic, the runway is named as 02 (or just 2 to our American counterparts) if you're on the south end looking north (well, north northeast, actually). Thats the number you'll see painted on the runway threshold as you approach to land. If you approach the runway from the other side, you'll notice a heading which is the reciprocal of 020°. This would end up being 180° off 020°, and that would work out to 200°, so the number on the threshold as you look south southwest would be 20.

For another example, a runway heading of 272° is normally named runway 27. It's reciprocal would be runway 09 (272 - 180 = 092, the '2' being rounded down)."

Not the cleanest explanation, but a starting point.