It is estimated there are 117 million travellers worldwide with frequent-flier accounts, but only 20% know how to get the most from their air miles. This is borne out by the number of unused frequent-flier miles - 8 trillion at the last count. If these were strung together they would stretch beyond Pluto and back.

The problem lies with perception. People who sign up to frequent-flyer schemes often don't see them as a cash alternative and use them recklessly.

Why blow 20,000 air miles on a trip from London to Madrid which can be done cheaply with one of the budget airlines? Many travellers don't even sign up for airline frequent-flier programmes because they feel they don't fly often enough. But more than half of all miles are now earned without leaving the ground, with airlines selling miles to credit card and telephone companies, car rental agencies and supermarkets.

Ravindra Bhagwanani runs a frequent-flyer consultancy in Germany. He says one popular misconception is that points earned with one airline must be spent travelling with the same company. "All major airlines tend to offer several schemes catered to different passengers," he explains. "There are so many airline alliances out there - One World, Star Alliance - that flying with one member of an alliance often gives access to seats on another member's flights."

The One World alliance covers nine major airlines including BA, American Airlines, Cathay Pacific, Iberia and Quantas. The Star Alliance has 15 members so far including Air Canada, BMI, Lufthansa and Spanair.

Bhagwanani helps travellers design a plan to make the most of frequent-flyer programmes. And the difference between some airlines and frequent flyer programmes is surprising. Take a passenger flying on a discount economy ticket between Heathrow and New York with BA collecting frequent-flier miles. Bhagwanani calculates it would take 28 return trips to earn enough for a free economy flight on the same route. The first airline reward scheme was set up by American Airlines 21 years ago. The concept came to the UK in 1990 when BA set up its frequent-flyer scheme which now has over five million members. BA saw a surge in the number of passengers booking on-line through the frequent-flier programme when the company simplified the scheme last July. The airline condensed five schemes into one - getting rid of 22,000 different rules governing the various schemes.

Richard Goodyear of BA says 2 million of the scheme's members are active, but denies the other members fail to use air miles because they are unaware of the benefits. "You may sign up for a type of card because there's a promotion on. The card is then never used. Our two million active members are regular travellers, loyal to us and know how to use their cards to best meet their needs."

Apart from airline schemes, there are other methods of building up points towards air miles. BA owns Air Miles, a loyalty scheme which gives points to shoppers at Tesco and Shell and NatWest credit card users. Air Miles is not to be confused with BA's frequent-flyer programme which is linked to BA flights. The Air Miles scheme acts as a travel agency for its members booking package deals or just flights and accommodation from 30 travel companies. Nectar is also a shopping-based scheme linked to Sainsbury, Barclaycard, Debenham's and London Energy.

There's no denying shopping to build up air miles takes time. A report published earlier this year by the consumer magazine Holiday Which? shows it would take over £230,000 worth of shopping and the best part of a lifetime to buy that holiday of a lifetime in Australia.

Through Air Miles a customer would need to spend £5,265 over 8 months to collect the necessary points for the same trip. A member of the Nectar scheme would need to shell out £5,850 over nine months. These figures are based on a person spending £45 a week in a supermarket, £20 a week on petrol and £300 a month on a credit card. Air Miles does offer promotions which allow points to be built up more rapidly. For example, a £100 shopping trolley at Tesco earns 16 points, but changing electricity and gas suppliers to the Scottish and Southern Energy Group earns 500 points. Claire Keane of Air Miles says its members tend to know how to maximise the number of points they can earn. "Our customers are very savvy," she says. "They shop at Tesco, use a NatWest credit card, they collect everywhere they can and come to us because they know they will get the best deal on a holiday. We offer the possibility of paying with a combination of Air Mile points and cash. If they use cash only we give them air miles in return at a rate of 1 mile for every £5."

There are a few simple rules to getting the most from frequent-flier programmes. The first is not to assume the company you are flying with is automatically the best one to save with. Hang on to air miles. The average active member in a frequent-flyer programme racks up around 13,000 miles a year. Save the miles for routes not covered by cheap tickets. And look out for special offers giving extra miles.

· For more details: www.globalflight.net, www.airmiles.co.uk, www.nectar.com