I don't know if it hurts you directly, but I do know that the total amount of available, unutilized credit can affect your ability to get additional lines of credit. Freakie Ed explained it better above.
In full, every month. Paying interest on a credit card is literally flushing money down the toilet.
Your credit score takes a small hit every time you apply for new credit. Having multiple cards is probably not the best idea for most people...choose one or two...I have one with a low limit/low interest rate for everyday type stuff and another with rewards/cash back set up for bigger purchases.
Auto loans, students loans, etc, are all things that will build your credit if you pay consistantly and on-time. Credit cards help too but aren't necessary. I still think its a good idea to use them if you can manage it - getting a card that pays cash back for things you would have bought anyway is free money.
Don't charge anything that you eat, drink, or burn. Things like groceries, restaurant meals, alcohol, cigarettes, gas....nothing sucks more than having a huge bill for a bunch of stuff that you have nothing to show for.