Preventing Rust on Hot Water Heater pipes?

I just replaced my hot water heater, basically because it leaked from the top where the copper pipe fitted onto the connection coming out of the tank (steel i think). We bought a "flipped" house and a lot of stuff wasn't done right. My current plumber replaced all appropriate pipes/fittings and put a whole new gas water heater in for us. My question is how to I prevent the rust on that connection? This plumber put what looks like a "collar" around the fitting but a little steel is still exposed...will rubbing vaseline on these threads help prevent rust? Sorry for the lack of the plumbing vocab.