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Any derogatories on your reports? You may simply need to grow your current accounts with CLI's.
Any savings to speak of? You might get a secured card from a CU like SDFCU - no hard pull that way. Any inquiries on your CR?
I would look at getting another credit line, preferably something thats a 'soft pull' so you don't have an added inquiry.
How much you put on a card is not important as long as its paid down before the balance reports. Thats usually a near the statement cut date. You can max it out and pay it down ten times a month and it won't effect anything as long as its paid down on the report date. Doing that may actually generate more transaction fees for the lender and actually encourage them to give you a CLI. I know if I was a lender I would see it that way.