To improve your score, first understand the breakdown of how it is calculated. Here's how your credit score is figured:
1. Payment History. This accounts for about 35% of your overall score. You get full credit in this area if you pay all of your past credit accounts on time, but if you only missed a few late payments, it still won't be detrimental to your score.
2. Amounts Owed. Maxing out your credit cards is a bad thing. Owing a lot of money on numerous accounts also makes you a higher risk borrower. This accounts for about 30% of your credit score.
3. length of credit history. How long have you had credit accounts? In general, a longer history means better FICO score because it shows you've been a reliable borrower for longer.
4. New credit. People today tend to have more credit and to shop for credit more frequently. But
opening several credit accounts in a short period of time can represent greater risk – especially for people with short credit histories.
5. Types of credit you use. A healthy mix of different types of borrowing (i.e. credit cards, retail accounts, mortgages) will help your score. This is worth 10% of your overall score.
Hope this helps, and check out the sources section for more information on how to improve your credit score. (I linked to an article titled 101 Ways to Improve Your Credit Score)
Lastly, keep in mind, Quizzle.com gives you a free credit score AND free credit report (the three major credit agencies only give you the report but not score), and you don't need to give any payment information. They also offer many credit improvement tools and online resources.
If you keep paying off credit accounts on time and building a good payment history, your score will naturally go up.