Credit scores are more like your driving record: They take into account years of past behavior, not just your present actions.

Close accounts you no longer use. A lot of unused credit you can access without further checks may negatively affect your rating. It can also make you more vulnerable to fraud.

To boost your score, "pay down your balances, and keep those balances low," says Pamela Banks, senior policy counsel for Consumers Union.

The best we got from the agencies was Experian telling us one in eight credit accounts is now from an energy, water or broadband and home phone provider.

Credit reference agencies used to make their cash by selling your info to lenders, now they flog it back to us, too. While credit files are crucial, the credit scores they sell have two flaws. Each lender scores you differently – so a universal score is a strange concept. And the score only looks at the info on your credit file.

One of the ways to improve your credit score: Leave old debt and good accounts on as long as possible, says Ulzheimer. This is also a good reason not to close old accounts where you've had a solid repayment record.

If possible, check files at all three agencies (or at least the big two, Experian and Equifax) as different lenders use different agencies – and don't assume the info will be identical on each.

For more information on this, including how to avoid it, see our Car Insurance guide.

In many ways this is the most important. Here, lenders obtain the crucial details of your post code, salary, family size, reason for the loan and whether you're a home owner.

You can apply at gov.uk. And don't worry, you can still opt out of the Open Register, which is the version that can be sold for commercial firms to use.

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3. It's about "will you make them money?" – not just risk

It's smart to stagger them, Sherry says. Send for 1 every 4 months, and you can monitor your credit for free.

For example, buy petrol on it once a month then pay off the bill in full.

Don't take it too seriously. Your credit reference agency credit file is important. Its credit score isn't.

If you have a poor credit history then there are credit-builder credit cards you can apply for.

Your credit score dictates whether you'll be accepted, whether you'll be given promotional rates, and the APR you'll be charged afterwards. See the Best Balance Transfers, 0% Spending and Credit Card Best Buys guides for full info.

Tough times mean banks are getting meaner and stricter about who they are prepared to lend to.

The solution to improve your credit score is to gather up all those credit cards on which you have small balances and pay them off, Ulzheimer says. Then select one or two go-to cards that you can use for everything.

Using this, we built a free Eligibility Checker – see moneysavingexpert.com/eligibility – which can currently be used for credit cards, but will soon be available for loans and hopefully more. It uses a soft search to map your score against lenders' criteria, and shows your percentage chance of getting different products, so you can hone in on the right card, minimising applications.

Medical problems you may have had in the past aren't listed.

They can kill your application. Access to too much available credit, even if it isn't used, can be a problem. If you have a range of unused credit cards and lots of available credit, it could be a good idea to cancel some of them. This lowers your available credit and should help.

After an error, it's possible to get successive searches wiped, but it involves negotiating both with the agency and the lender, and it isn't easy.

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If you need to boost your credit score, it won't be easy.

Imagine a bank wants new mortgage customers. That's a costly sell. Instead, it offers a current account paying a high rate of interest on a small amount kept in it. Yet, when you apply, rather than scoring you as a bank account customer, it could actually be scoring to see if you're likely to be a profitable mortgage borrower in the future – you might face rejection if you aren't.

The one exception is withdrawing cash on a specialist card abroad. See Overseas Credit Card ATM Withdrawals for full info and why they're not too bad.

Yet the good news is that these searches are 'soft' searches, so other lenders DON'T see them and they won't affect your credit rating, Only you can see the search has been done.

Keep personal details the same between applications. It's crucial to be consistent, even over long periods, when you fill in application forms. If you have a number of job titles or phone numbers, try to use the same one on every form. If you use different ones, you might be flagged up by fraud scoring.

If you're planning a major purchase (like a home or a car), you might be scrambling to assemble one big chunk of cash.

To check the info it holds on you, write to it making a data protection request and enclosing £10. The National Hunter website explains this. This can also be a useful thing to do if you think you're a victim of ID fraud.

How to improve credit score

This is done with a massive system of automated impersonal credit checks. It's often far cheaper for a lender to reject some people who it should be lending to than it is to accept some it shouldn't be lending to.

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If you are in difficulties, the cliché "contact your lender" is a good one. Hopefully it will try to help. Changing your repayment schedule is preferable to you defaulting – and though it will hit your credit score, it's better than a county court judgment or decree against you.

Simply follow the instructions online – it'll ask you a series of questions aimed at identifying you, and the local electoral borough you need to register with. Note that you'll need your National Insurance number to hand.

So if you fancy a cashback credit card and have no other credit you need to apply for in the next six months or so, great, spend your application. But if you're just about to apply for a mortgage, wait until after you've done that. Prioritising is important.

These use a soft search (so you see it on your file, but lenders don't so there's no impact) to show your odds of acceptance for the top cards (and loans), so you can hone and minimise your applications.

The reason this strategy can boost your score: One of the items your score considers is just how many of your cards have balances, says Ulzheimer.