If you currently have a high debt to credit ratio, you might have difficulty raising your credit limit. (creditsesame.com)
And like you say, a low debt to credit ratio and keeping your oldest credit are a couple of things people may not know about. (freefrombroke.com)
Try to avoid using more than 50 % of your available credit (preferably less than 30 %) to maintain a good debt to credit ratio which will help increase your credit score. (creditabsolute.com)