Fly mid-week
Book your flights midweek; it is not only cheaper to fly Wednesday to Wednesday than over the weekend, it is also cheaper to book during the week. Most airlines bump their prices up at the weekends because they know this is when most people have the time to research and book holidays. Come Monday, booking prices drop again.
Choose your budget carrier wisely
You might be able to find a £1 return fare with the low-budget carrier Ryanair, but make sure you maximise it by using the right card to pay. Most credit cards will incur a payment handling fee of £5, and that's per person, per one-way flight! If you are travelling as a family, that ultra-cheap flight can come to seem less of a bargain.
Also, most budget airlines do charge for carry on or checked in baggage; weigh the pros and the cons to see if it is not better to simply go with a standard airline.