In my experience you need to be 'with' the company for about 12 months and then simply call customer service and request a credit limit increase. If you've paid all your bills ontime and don't have a balence or are carrying a low one, then they;ll will typically agree to your request.

This also works well for getting out of things like yearly membership fees and lowerign your APR. Call customer service and complain about your issue. Throw in that your thinking about cancelling your account as well. They will try to retain your account if its a good one and that means some supervisor bending over backwards to offer something that will keep your business.