I'm obsessive about my credit rating now as well, because I'm looking at buying a house in a couple years. The best thing to do is to pay your bills on time, every time. Pay off your credit card balances in full every month, or at least try to pay more than the minimum. It's gonna take some time, but eventually your score will creep up.
I'm afraid you might be in a jam with the company that you paid off. It would have been better to negotiate something before you paid them off, because now they have no more motivation to help you out. It is my understanding that they CAN request the credit bureau to erase something, but they probably won't now since you've paid.
If you haven't already, pull a copy of your credit report, and first make sure that there are no errors on it. Then you can get in touch with the credit bureau yourself and if the bill you paid off is still showing as "unpaid", you can file a dispute, and the credit bureau will look into it. (Just a tip: I would file a dispute on any bad stuff that is really old. I did this, not knowing whether I had a valid dispute or not, but I got some old stuff taken off of my credit report, which really boosted my score!) You can get one free report each year from www.annualcreditreport.com
There's also a lot of great info on www.myfico.com I subscribe to their Scorewatch, and it lets you keep up with your credit score, gives you alerts on when it changes, etc. This way you'll know how whatever you are doing is affecting your score.
Hope this helps!
