Wow, that is a great suggestion, I never thought of that. Tucker will (usually) come when I tell him to. Outside he doesn't wear a leash and if he is starting to wander or doesn't come inside when I'm going in, I have this certain tone of voice I use and say "get over here you" and he comes every time. But, I'm sure that's not what you are looking for!
I think aqui is a great idea.