When you meet a prospective partner/date/booty call, make sure you both understand what sex means to each person : oral sex ("oral-walk-in-the-park"), PIV ("I can do anything except PIV with you"), different gender, different rules ("girls don't count") and other variations. Confusion about this doesn't work out well.
Before you start getting serious with a prospective partner, make sure you both understand what kind of poly relationship you both want/expect/hope for. Don't agree to be a long term secondary if you want to be a primary instead. Don't agree to a 'don't-ask-don't-tell' if that isn't what you want. Misleading someone or being misled doesn't work out well, either.
Work your ass off to get to know your OSO. Try to get along even if it kills you. Suck it up.
Remember, boys are dumb
Treat your primary best. Even if you're all wrapped up in NRE. Empty out the fish tank, clean it, fix it up, and set it back right; don't just sprinkle fish food into it on your way out the door.
Always get a 'signed field trip note' from your date's SO before you get it on. "My wife always says it's ok" or "My husband doesn't mind me doing whatever I want" doesn't cut it.
Being poly means talking about your feeeeelings. At length and at depth. It means communicating, negotiating and scheduling continously. It means overcoming your jealousies, and working through conflict. If you can't do any of this, see a shrink and don't be poly.
Labels suck, but they ease your way into a new relationship: "I'm bi", or "I have a primary, a secondary, and an LDR". Make sure you both understand your different definitions. Here's an example of a poly hierachy: You come to an agreement with your primary to move to another city; you discuss moving to another city with your secondary; you tell your fuck buddy that you're moving to another city.
Nasty poly breakups usually have deep repercussions throughout your poly community. Try to keep things civil at all costs.
Children change everything. Always put them first. When kids are involved, move heaven and earth to keep them in a stable poly family.