1. Not pranking locally is a good start to not getting in trouble. This way, more than just the local police has to be involved if anyone complains. Having to deal with multiple police departments and/or FBI complicates things greatly for someone wanting to complain. I've been contacted by police in other states before and I'm just an asshole to them since there's not much they can do.
2. Don't make threats of any kind. Don't do anything that involves the police or other emergency services. No bomb threats or attempts at fraud. Keeping things silly and innocent makes the police not care about prank calls.
3. Hide your phone number. *67 is sometimes good enough, but hiding it even further can make it impossible for the victim to figure out who called them. My favorite method these days is using Skype since it doesn't display a caller ID. Before this it was Vonage with an out-of-state phone number that would cause people to call the police in the wrong state. Most types of internet based phone services can be a good way of concealing yourself. MagicJack doesn't require any of your real information. Spoofcard is awesome for pranking because you can lead them down a completely wrong trail.
