In regards to Revolving Credit (Credit cards, etc.):

Pay down the balances to within 20-30% of your total credit limit. This should help to boost your scores. If you do not have the money to pay this down, call your credit card companies up and ask for a limit increase... this will automatically allow your debt to limit percentage to lower. If you do not have any revolving credit, get some! Dont be afraid of credit cards. If used properly they are very effective tools!

In regards to Installment Loans:

If you have ANY loans with high risk lenders (i.e. American General Finance, Well Fargo Finance or any other company with "Finance" in their name) pay them off quickly, or borrow from your local bank / credit union to pay them off as soon as possible. Due to the risky nature of these lenders, the fact that you have this type of loan will hurt your scores. Paying them off will certainly boost your scores.

In regards to credit mix:

Make sure you have a balanced mix of types of credit... in other words, you should have 1-2 credit cards, 1-2 loans and possibly a mortgage. This type of credit mix shows good balance in credit. With a good payment history, a healthy score will be right around the corner.

One last tip:

Although the big 3 credit bureaus claim it does nothing to improve your score, mortgage lenders across the US swear that they have found that by requesting to opt out of the credit bureaus mailing lists (that they sell!) you may see a boost in scores in about 10 days... usually small, but it might push you over the goal you need. The three major credit bureaus have one site in which you can request to be opted out of the pre-approved offers. That site is located at optoutprescreen.com.