FICO is an unpredictable beast, but really I don't think you have anything to worry about. Your best strategy for keeping FICO well-fed and happy is:
1) Have credit cards, but keep just enough of a balance to get a bill each month. If you're using more than 25% of your available credit on any one card, make paying that down a priority.
2) NEVER pay late. Automate payments if you can. I have Bank of America, and I can set up automatic payments for the minimum payment due a week before the due date, every month, forever.
3) Save money to buy that car. Finance part of it in a simple-interest loan. Pay the loan off ahead of schedule (put an extra $100 a month or so toward the principal). A paid car note looks good on FICO.
4) Periodically apply for a new credit card, and credit line increases (not that you'll USE the extra credit, it just looks good).
Do these things, and your FICO will rise. You're just fine for your age.