another idea, is to make a set to catch a particular animal. i know non targets will always happen, but if you want to catch a fox in a certain set, try and figure out how to catch a fox, not just anything that comes along. if you want to catch a bobcat make a set for a bobcat and try to learn as much about that animal as possible.
i'm happy with a bobcat in a coyote set, or a fox set. but i hate it when a 10 dollar coyote messes up a 75 dollar cat set, lol. or a 5 dollar coon gets in fox set.
some animals you can't avoid, one of them is a coon. they weigh at least as much as a fox or even bobcats so even pan tension won't keep them out. but as far as possums go, i hate them and avoid everyone of the stinkin grinners i can, lol. i do that with pan tension.
for the most part coyotes shy away from flashy type cat sets ( i know not always) and you can keep them out.
try this and if you pay attention to detail and the little things, such as tracks, and asking why did the animal step here or where did he come from, check scat to see what they are eating, all these things will help you understand the animal better and in turn will make you a better trapper. one big thing tho....you must have patience.