A little problem

Recently...I found a little problem in my yard... I always find dog's poop in my front yard... thanks to those people who don't pick up their dog's poop. It pisses me off. (you know I don't get pissed off that often) I don't mind if dog poop in my yard, but just pick it up. The worse, i don't know whose dog did it... tons of dogs walk pass my front yard everyday. What should I do? I was thinking making a sign, and say "Potty train your dog, or else pick up their poo" or a sign say "no trespassing, not even dog." what do you think i should???