I think it's really based on personal opinion. I'm not too great at park, but I don't think I need a park instructor to tell me how to do boxes and rails. Just practice your balance on small stuff at home. For example: Rail road ties, tires, crates, tables, fridges, whatever's available to you. and try the stuff you want to try at home and then step it up bigger and bigger. and then once you've got a trick down, then try more at home and try it again. It sounds lame, but start small, it helps.