Holding Huge Balances - Whether you have met the debt limit or not, only making minimal payments and holding on to that balance for a long time could hurt your credit score.
Paying at the Wrong Time - Even if you pay off your card every month, great. But not paying it off at the right time means you could still have roll over credit debt--which looks like you still have a balance, not great.
Closing Unused Credit Cards - If you aren't using a card, you think you don't need it. Creditors want to know how many lines and if you are effectively using credit. Keep it open even if no balance.
Applying too Often - Don't let everyone run your credit often. This may appear as you are trying to run up too much credit which could appear as a risk to creditors.