There's pretty much nothing I can do. One competitor even made an icon almost identical to mine. I contacted Apple, but all they did was forward my complaint to the competitor. The competitor first sent me a threatening email, but then they changed their icon.

I think the best way to guard against competitors is to improve your product. The first one and a half years I was always far ahead of my competition. Since about a year I haven't had time for any significant new features, so my competitors have catched up. Some competing apps now have more features, some are a lot cheaper... Unfortunately the next logical steps for improving my app are so much work that I don't know if they would pay off.