How can you improve your credit score?

It's virtually impossible to change your score in the time between when most people decide to buy a home or refinance their mortgage and when they apply.  So the short answer is, you really can't "on the spot."  But there are strategies you can live with to make sure when you apply for a loan your score is as high as possible.

Make sure that the information each of the three credit reporting bureaus has on you is consistent and up to date.  Order a copy of your credit report a couple of times a year, review it thoroughly and determine any inaccuracies.

This won't hurt your credit scores because you're the one pulling the report.  It's only when a third party (like me) pulls a credit report at your request for the purpose of considering extending you credit that your scores are dinged. 

Step #1 for Improving Your Scores
Limit the Inquiries on Your Credit Report. 
Don't respond to credit offers you receive in the mail.  Those unsolicited credit card solicitations in the mail don't count against your scores because you didn't request the review those solicitation are based on.  They are based on "skimming" consumer credit files for basic criteria and the offers are their way of "fishing" for prospects.  It's when you respond and say "Yes, I'm interested" that you are requesting the solicitor pull a thorough credit report for the purpose of extending you credit.  So, the key to these solicitations is to not respond if you're preparing to obtain a mortgage (or buy a car or borrow money for a student loan, etc...).

Note: Theoretically, if a series of credit reports is requested on your behalf during a limited amount of time (especially for the same purpose), your score goes down until time passes without any inquiries. Changes in the law though have made "consumer-originating" credit report requests not count so much. Also, a series of requests in relation to getting a mortgage or car loan is not treated the same as a number of credit card requests in a limited time. This is because the credit bureaus, and lenders, realize that people request their own credit reports to keep up with what's on them, and smart consumers shop around for the best mortgage and car loans.

The two main components of your credit score are your payment history and the amounts you owe.  A payment history with no 30, 60 or 90 day lates will reflect well on your scores.  Late payments work against you, so it's extremely important to pay bills on time, even if it's only the monthly payment.

Dont "max out" your credit lines.  Since the size of the balance on your open accounts is a factor, lower balances are better.  Keeping your balance - the amount you owe - on revolving credit (credit cards) at 75% of the limit or less has a positive impact on your scores as does keeping a small balance rather than a zero balance.

Bankruptcy filings and foreclosures, which can stay on your credit report for as many as 10 years, can significantly lower your score. It's never a good idea to take on more credit than you can handle.

It's said that by carefully managing your credit, it's possible to add as much as 50 points to your score.