The scoring formula seems to like seeing 3 - 5 cards, maybe 3 bank cards (have Visa, MC, Discover, Amex) logos and a retail card (a store or gas card with NO bank card logo --no Visa, MC, etc.)
It's also pretty smart to diversify your credit these days.
You just have the one card, then? Any open installment/ mortgage credit, etc.?
I would think about what would help your overall financial picture: rewards cards, etc. Do you drive a lot, and a good gas rewards card would help? Are you remodeling, and a home improvement store card would help? --and so forth. Don't just get a card for the sake of getting a card; make it useful to you.
You might want to make one of the bank cards from a credit union. They seem to be far and away the most sensible and relaxed lenders these days. Make another one from one of the Big Boy banks --BofA, Chase, Citi, etc. If you get a store or gas card, read here first to make sure that it's not really a bank card, and get one from a place where you actually shop, and that will reward you with early-bird sales and so forth. Check the issuer of the card: some great stores have dreadful cards, because of the bank that handles them.
Anyway, if you do get additional cards, I would keep only one reporting a balance, and have the others report $0. Do use them, in order to make the issuers happy, but pay them off online lines several days before the statement is due to post, so that it will show a $0 balance. Not only will this help your scores, but it also reassures all your lenders that you're not suddenly going on a mad credit craze, running up balances for some reason that you might not be able to pay off.
You might want to read the FICO High Achievers thread in my siggy. We've collected the positive and negative comments that people get on their score reports in an attempt to paint a picture of the kind of credit consumer that the scoring formulas like.
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007