My cat is leash trained. The trick is getting them used to a harness early. Once that's done it's pretty easy to take them out. The only hard part I've had is getting him to come back inside. We usually go for walks about once every 2 weeks or so just to get him out of the apartment. He knows where his harness is and gets all excited when I bring it out and will even paw at the door if he wants to go for a walk. I get some strange looks walking him, but he loves it. He even climbs trees though I don't let him get too high out of reach.

Peace