Great Question! I am answering this with the assumption that you have seen your credit. Here is where I would start...
Dispute Errors. If you find something that doesn't make sense or is wrong, contact the agency reporting it to dispute it (i.e. Equifax / Experian / Transunion).
Pay Down Revolving Lines to 30%. What this means is that if you have a credit line of 1000 the best balance to have on that credit line is 300. Credit Cards, Store Cards, and Credit Lines are the ones that will affect you here. Take your Limit and multiply by .30 and that is the balance that you want to have on that particular item.
Make sure your credit is active. If you have cards open, use them. Pay them off. Active shows that you can have a loan/line and make a payment.
Ask for credit line increases. This will help with the second item of paying down debt as it helps make your ratios to debt and income lower which is better for you!
If you need any help I have some resources that would go deeper with you here!