A credit score is either a blessing or a curse cast down on you from the digital cloud.A score of 740 qualifies you for the best interest rate on a conventional mortgage. A score under 640 means ugly interest, and a number under 620 makes it hard to get a mortgage at all.This measure of your worthiness -- generated by a soulless computer -- also affects the deals available for car loans, credit cards and auto insurance.But you have ways to nudge that score higher.Such tweaking is part of George DeMare's business. He is managing partner of Midwest Mortgage Capital, a mortgage lender in west St. Louis County, Mo."I deal with people with 640 scores all day long," he said.And he looks for ways to push them up.Lately, he has taken to lecturing on the mysteries of scoring. His main advice: Pay your bills on time and borrow sparingly.That's the best way to fix credit over time. But if time is short, you can make other maneuvers.First, a word about scoring: Most of the blessing, as well as the curse, comes from the king of credit scoring, Fair Isaac Corp. The company, better known as FICO, provides the software used by many major lenders and credit-reporting companies. FICO uses five variables for scoring credit. Its main rival, Vantage, uses six.With a FICO score, on-time payment is the biggest factor, accounting for 35 percent. The amount owed influences 30 percent of your score; the length of your credit history is responsible for 15 percent; the type of credit used and recently added credit each affect 10 percent.On-time payment is also the biggest factor with Vantage, accounting for 32 percent.Scoring formulas are tailored for different lenders. If you buy your score from a credit company, know that it may not be the score that your lender sees.The scores are derived from your credit report. The report tells how much you owe, how close you are to maxing out your credit lines and whether you pay on time. The report also scans public records for bankruptcies and court judgments against you.So the first step in boosting a score is to make sure the credit report is accurate.It's often not.In a recent Federal Trade Commission study, a quarter of consumers found errors on their reports that could affect their scores.A clerk's finger slips on a key, and somebody else's debt is reported as yours. Court judgments often stay on reports after they've been paid off, DeMare said.Collection agencies are not diligent about reporting satisfied debts.You're entitled to a free report once a year from each of the three major credit bureaus -- Experian, TransUnion and Equifax.Go to www.annualcreditreport.com or call 877-322-8228. If you're denied credit because of your report, you're entitled to another free one.If you find a mistake, contact the reporting agency. The Federal Trade Commission found that 4 in 5 consumers who complained obtained a change in their report.Credit-reporting agencies look at the amount you owe and at how much of your available credit you are using. The smaller the debt, the better. If you have a $10,000 limit on your credit cards and you owe $1,000, that's OK. A $9,900 debt is awful. FICO likes to see debt at 30 percent or less of available credit, spokesman Anthony Sprauve said.If you can't pay down the debt, call the credit card company and ask for a higher limit, DeMare said. That will make your debt percentage smaller.Shuffling debt around can also help. Suppose you have two credit cards with $10,000 limits, with $9,000 owed on one and nothing on the other. Moving half the debt to the unused card will help, he said, and the FICO spokesman agrees. The formula judges you on how close you are to maxing out a card.