Pay them off every month if you can. At the very least, keep the balances under 1/2 of the credit limit. Just use them fairly regularly for SMALL things to keep the accounts active.

Mortgages and car payments are great forms of credit, but you don't have to have them to have good credit. 3-5 tradelines with good history is what you are aiming for. The longer you maintain good credit, the better it is, but your score can raise significantly in 30 to 90 days using the methods I outlined earlier. You don't have to wait a year to see big changes in your score.

Also, when you are shopping for a loan (mortgage, car, etc) you will want to keep your searching to a minimum so your report doesn't get pulled tons of times to compare programs. Go to a broker (like me ) who can pull your credit once and shop for you with multiple lenders.