Approximately 17~18 years ago I built my first "real" railroad in N-scale (I took it beyond flat plywood and fully scenic'd it). I cut my own stencils out of a plastic milk jug container with an X-acto knife (or any other similar plastic container would work, too). I then stipple-painted the markings on the "asphalt" with a stippling brush and cheap craft paint. Like Ron045 and airbrushing, it produced a slight weathered look which I really like.
Here is a picture of a few N-scale road markings made with home-made stencils. I will most likely be doing some O-gauge road markings sometime yet this year, with the same method.
