From personal experience, I have found the best way to keep a good feedback rating is to pay on time.
Sellers expect to get paid within the time specified, if you are having problems, e.g, computer is playing up, etc, then let the seller know. Don't leave it until the last minute, or until there is a dispute to contact them.
Most sellers are understanding and caring people, you do the right thing by them and they will do the right thing by you.
Just remember, you can't walk into most shops and buy things without paying for them, why should eBay be any different.
Ebay has been a very pleasurable experience for me, and can be for anyone with a little common curtisy.