If you're about to book a holiday, think carefully about which day you plan to leave before you book the trip. According to research by Which?, Tuesday is the cheapest day of the week to fly from the UK. For example, flights with easyJet from Gatwick to Alicante on a Friday were, on average, 35% or £28 more expensive than Tuesday flights. The consumer association, which looked at 1,174 September flights to three destinations in Europe with three airlines – British Airways, easyjet and Ryanair, found that Sunday was, on average, the most expensive day on which to return home. For example, return flights with easyJet from Gatwick to Alicante on Sundays were on average 45% (£56) more expensive than on Thursdays. As well as cheaper days, there were cheaper times of the day to fly, though this varied across airlines. For example, BA's cheapest outbound flights were in the morning (before 7.30am), but with easyJet the mornings (between 5.45am-11am) were their most expensive time.

Here are some more useful tips for keeping travel costs down. Please add your own money-saving advice to the comments below:

Make the most of cashback websites

We all understand that if you book a holiday through a travel agent, the agent earns a percentage of the total cost as their commission. It's the same online: if you click a link on a website and make a purchase, the referring site gets to keep a certain amount of what you paid. Cashback sites such as Quidco.com and TopCashBack.co.uk flip this on its head: they provide personalised referral links, collect the commission, then pay the money back to you.
Quidco is currently offering 11% rebates on hotels booked with Eurostar, 7% on hotels booked through Lastminute.com and 5% on hotels booked through Opodo – just for clicking an extra link.
Example Click QuidCo link, book a £150 Eurostar hotel room and receive £16.50 back

Avoid booking fees on UK train travel

All UK railway ticket websites use the same database so offer the same range of tickets and prices as their competitors. Despite a thetrainline.com is no cheaper than its rivals – but unlike other sites they charge a £1 booking fee per transaction and an additional £3.50 if you use a credit card. Instead book your tickets through the train operators' own websites. South-West Trains (southwesttrains.co.uk) doesn't charge fees and even offers a 2.6% rebate on all tickets nationwide if you book through a cashback website.
Example York-Edinburgh off-peak return, booked through thetrainline.com with a credit card, £85.50. Same ticket booked through Quidco and South-West Trains, £78.90

Swap flights for the ferry

Want to escape at short notice but find flights too expensive? SailRail.co.uk is a little-promoted ticket left over from the days of British Rail. It enables you to travel from any UK railway station to Dublin for a flat rate of £43 each way, with all rail and ferry connections included. Simply turn up at your local station, buy the ticket and take the next train to Holyhead. You have an unlimited luggage allowance, there's no check-in and a beautiful train journey is included for free. A similar service, the Dutch Flyer (stenaline.co.uk), allows travel from any London station to any station in the Netherlands for £39 each way.
Example Depart London Euston at 7.10am and arrive in central Dublin before 2pm, £43 booked on the day. Equivalent flights and airport transfers, over £130

Make the most of local deal sites

We're used to seeking out discount coupon deals at home but few people check what's on offer abroad. In the run up to your holiday keep an eye the foreign versions of Groupon and other coupon sites to grab a bargain. You could discover where the locals eat, save 50% on meals and gain discount access to popular attractions.
Example Groupon New York is flooded with offers for substantial discounts in local restaurants, such as a $40 meal for $20 at Lantern Thai in the East Village. The French version of the site is offering a helicopter flight over Paris for two for €189 instead of €480

Split-ticket your train journey

Even if you haven't booked a UK train ticket in advance you can make substantial savings by split-ticketing. This exploits the fact that two tickets split at an intermediary station can be cheaper than buying one ticket all the way to your destination. This is perfectly legal as long as the train stops at the intermediary station. Train ticketing websites such as splityourticket.co.uk help spot combinations that may work.
Example An off-peak day return from Manchester > Bristol is £75. But a series of day returns from Manchester-Stafford (£20), Stafford-Wolverhampton (£5.80), Wolverhampton-Cheltenham (£22.30) and Cheltenham-Bristol (£8.20) are valid on exactly the same trains but cost only £56.30

Split-ticket your flight

Splitting tickets also works with no-frills airlines. Airlines such as Ryanair often sell off large numbers of cheap flights to unpopular destinations at a flat rate of £10 or under. Rather than take a direct flight. book two flights via an unpopular destination, leave an hour to transfer and reap the rewards. But there is a risk: if one flight is substantially delayed and you miss your onward journey, you're on your own. Watch the website for flight sell-offs.

Use a card that doesn't charge abroad

There's no point in scrimping on your travel costs only to run up charges on exchange rates and card purchases. Most high street banks add fees to transactions made overseas and give a poor exchange rate. Consider getting a credit card exclusively for use abroad – Money Saving Expert recommends the Halifax Clarity card, which gives you the real exchange rate.
Example A purchase in a foreign shop on a standard Lloyds TSB debit card carries a £1 transaction charge and a 2.99% foreign exchange fee. The Clarity card charges the standard Mastercard exchange rate and no extra fees

Stay in someone's house

Sites such as as the popular Airbnb.co.uk allow strangers to market a spare room or apartment for rent and provide you with the security of an eBay-style feedback system. Prices are often lower than in hotels, the rooms in better condition and the welcome friendlier.
Example A double room in a central Berlin apartment for £29 a night (airbnb.com/rooms/172374). The owner can recommend places to go and even offers a massage at the end of a long day's sightseeing

Buy a local SIM card for internet access

If you're going away for several weeks, especially outside the EU, avoid internet cafe charges and mobile data roaming fees by buying a local pay-as-you-go SIM card. Fierce competition among mobile operators means data deals can be even cheaper than in the UK.
Example 59,000 Colombian pesos (£20) buys a SIM card from the Tigo network that includes 3GB of mobile internet – more than enough to check your emails, research travel plans and send a photo back home, see prepaiddatasims.com/prepaid-phone-plans-in-colombia for more details

Compare, compare, compare

Before booking your holiday ensure you can't save money elsewhere by using a "scraper" website to compare prices from hundreds of airlines and travel agents. Momondo.com provides a brilliantly designed calendar summary of flight prices that allows you to spot and avoid peak periods. And Kayak.co.uk comes with a fantastic set of apps for iPhone, Android and BlackBerry.
Example Search for a one-way flight from London to Istanbul in February on Momondo and at a glance you can see that prices vary between £40 and £107