This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
The short answer is, clean them the same way you would a pressed CD. Take a
lint-free cotton cloth and wipe from the center out. It's important to
move in a straight line from the hub to the outside, rather than moving in
a circular motion. The act of cleaning could cause the surface to abrade,
and the error correction employed is better at correcting scratches and
marks that go from the center out.
You have to be a little more careful with CD-Rs than you are with pressed
CDs, because the lacquer coating may not resist certain chemicals as well.
Some CD-R discs all but fall apart when exposed to alcohol. Some really
cheap ones start to dissolve in tap water. Your best bet is to just use
a dry, clean, soft, lint-free cloth, like you would use to clean the lens
of a camera.
(In practice, a wadded up tissue works pretty well, but it's best to avoid
paper products. Lens cleaning papers are great for glass, but polycarbonate
is much easier to scratch.)