Trapping is a lot of fun but also a lot of work. Set on sign. If you're on target, you won't need very much flagging. Usually, flagging is just to get the cat in the area if you're not sure where to make your set or you're not sure where the cat is traveling. Try going to wal-mart or hobby lobby and get some googly eyes and put them at the back of one of your sets. The biggest Tom I've ever caught was on a walk through set with a tuft of a coon tail wrapped up in a piece of bailing wire and stuck in the ground where the piece of tail was about two inches off the ground. No flags or anything. I mostly use MB650's.
I also like flat sets for coyotes like azcatter said. A good cat gland lure works good on dogs. Also, Slim Pederson has some good cat trapping books and videos.