What sort of lead up work?

I've done this with a electric collar but while he understands not to follow-up snake scent he still shows too much curiousity. One of his favourite games is to hunt lizards but he knows the difference between lizards and snakes. Eg he finds a lizard's scent he follows it. He finds a snakes scent and he jumps, but too often still follows up a few too many steps.