Comment. This isn't a rule I'm going to rabidly enforce or anything, but it's just a nice thing to do. I put a lot of work into my icons, and I'm trying to improve, so feedback is my life blood. I understand that sometimes you may be in a rush, but please take the thirty seconds to write me a little note whenever you can.
Don't hotlink. Hotlinkers go to the special hell reserved for evil, lecherous humps and people who talk in the theatre.
Don't alter any icons (bases excluded) without my express permission. I'm usually open to re-interpretation as long as you ask me first, but it's bad form to just grab someone else's artworks and do whatever you please with them. If you need help altering something, you can ask me, and chances are I'd be more than happy to do it for you.
Enjoy. I make these for that purpose, and nothing makes me happier than seeing someone enjoy my icons. =)