Oh, and one more important thing: try never to cancel a card. Especially if the card has a long and clean history. I made the mistake of believing the old "too many cards is bad for your credit" adage and canceled a couple of old cards I never used but then came to understand I just shot myself in the foot by losing a nice chunk of two of the most important things you can have in your credit report: History and Quality. It doesn't all drop off your report immediately upon cancellation of course but still, I could have had that nice history safely there for a lot longer.
Now the only reasons I will cancel a card is if I don't want to pay an exorbitant fee (which usually means there isn't even a year of history to lose from the card anyway) or if a credit card issuer tells me to close one in order to get another one I want or I know beforehand that that's going to be the case. Otherwise I keep all the good history I can for as long as I can. I'll even use an oldy, moldy card once in a while just in case the credit card issuer might get the idea of closing a card on their own due to it not being used in the last 10 years.