1. Know what's on your credit report. Call and challenge anything that's an error. Look at it at least once a year.
2A. If your score is low because of too much debt, missed bills, and etc., look into what accounts you can close so that at least you can't acquire more debt. Work on paying as much more than the minimum bill as you can in order to get things paid off.
2B. If your score is low due to lack of activity, get one good all purpose credit card (preferably from somewhere like your credit union or bank) with a small credit line. Use it to buy something you're going to have in your budget anyway, such as fuel for your car. Use the budgeted money to pay it off every month.
3. Work out a system to make sure your bills are paid on time and in full. A couple of years not missing a cellphone payment does more than you think for your score.