The day you make the final payment on the credit card, do it over the phone with the bank, and tell them you want to pay it off and close it. They'll ask why, and you tell them your limit isn't high enough to merit having it. Take whatever limit increase they give you, and go apply for 2 more dinky credit cards.
I don't fully understand the rating system, or how they offer what to who, but when I started getting $500 credit cards, it seemed even if I didn't use them I was being offered more of them. Then when I had so many banks willing to give me credit, my bank offers me a 50K line of business credit just because. The more banks that are shown to trust you with any amount, the more willing the next bank will be.