• Review your credit report and correct any errors you find. Getting rid of inaccurate (and bad) information can sometimes improve your score dramatically.
• Advice used to be given to close old and unused credit card accounts in order to reduce your “potential” available credit (which could change your debt ratio after you’ve been approved for a loan). Now, however, the ratio of your debt to your credit limit is more critical, so closing old accounts only raises that ratio – which you don’t want to do. Some people have moved debt from several credit cards to one card and then closed the old accounts. Since creditors look at the debt-to-credit limit ratio this can have a bad affect on your credit score because you have the same amount of debt but less available credit. So don’t close old credit card accounts just because you’re not using them.
• Creditors also now look at the average age of your accounts so, again, keep those old accounts.
• Reduce your balances on credit cards to 75% or less of your available credit (25% is preferable).
• Pay your bills on time. (This is probably the most important of all!)
• Don't let anyone make an inquiry on your credit report unless you absolutely have to. The more inquiries, the lower your score.
• Don’t open new credit card accounts just to increase your available credit in the hopes of raising your score.

Also, remember that some improvements -- such as better efforts at making payments on time -- may take time to impact your score. So, time is also a factor.

-from www.howstuffworks.com