That limit is over 3 cards -- Visa, Mastercard, and Discover. Discover I hardly use, but they were the only ones that would offer me credit when I needed it at 17, so I stick with them out of loyalty.
I put major purchases on them, like tuition, travel, etc, and pay it off over a 3 to 4 months even if I have the money to pay it off. The cards usually prefer if you actually don't pay it off immediately. If you pay it off at the end of the month, not only do they not make any money on the deal, but you really didn't use "credit" you probably just used your card in place of money you already had. Paying it off over 2 or 3 months shows that you can actually borrow money and pay it back over a period of time without missing a payment.
It's worked for me. I have an exceptional credit rating. When I got my apartment some time ago and they ran the credit check, the leasing agent said he'd never seen anyone have as high as score as me, let alone someone that was only 21.
