I make around $2000/month from two Mac apps (database tools). Revenue is falling as multiple competitors cloned my profitable app, so I have to do more client work again :(
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.