The Pro Book by Bob Henning has a section with a set of standard safeties. They are a good place to start. There is also a section on kick-safeties ... which are good to know after some farging bastage plays a safety on you.
As I don't get to play as often as I'd like, I don't really practice playing safe anymore. It's more like that I remember my safety attempts during a game that went wrong and set them up again afterwards and look what I did wrong and what I should've done better.