Tip One: If you can't say anything nice....

Keep your mouth shut. This is one of the most important things you will ever need to understand in this business, no matter your skill level. It doesn't matter if you play in your parents' garage with a few friends, or if you're touring the world with Metallica, never say anything bad about anyone. You never know who may at some point be able to impact your career for better or worse. For the sake of understanding, imagine you grew up in the same area as Lil Wayne and spent the majority of your time putting him down thinking it would push your career ahead of his and you might steal some of his fans. Somehow it actually worked and a few years later you approach a major label in hopes of a deal. Only one problem; Lil Wayne was signed to the same label several years before you and he's a best seller. If they have to choose between the two of you, Wayne wins. You try to get a good venue but Wayne is there all the time and brings big crowds. The venue won't risk damaging that relationship and won't have you perform. Again, Wayne wins. Granted these are extreme situations and are not as likely to happen to you, but regardless are real concerns in a list that can affect your career. Support your fellow artists, they want it just as bad as you, and if you help them out chances are good they'll return the favor.