Flights
I'm a fan of the low cost carriers. I have used Ryanair on many occasions and found flights for as little as £20 return including taxes if I travelled midweek in low season. My only advice about using low cost carriers is to be wary of booking indirect flights. This is because they operate as "point to point" airlines. This means that if you miss the connection you've had it. You either have to book an earlier flight and hang around the airport for hours or pay to stay overnight near the airport. Also some of the airports used by the low cost airlines can be pretty far from the cities they claim to serve, so you should always check this out before booking.
The major scheduled airlines also offer some great fares and if you do miss a connection they will sort out your onward journey for you, as long as you have booked a multi leg journey with them. Fare comparison flights allow you to find the best deal. There may be other factors as well as cost which are important to you, such as flight times. You may prefer to pay a few pounds more for daytime flights to avoid a middle of the night drive to the airport or the additional expense of a stay at the airport.
Kayak.co.uk has a good reputation for finding the cheapest flights by searching more than 100 sites.
I like wegolo as it compares the prices of all the low cost airlines. It offers the possibility of using a different airline for you outward and inward journey depending on the price and flight time. There's a "Hot & Cold" search option where you select your desired weather conditions and it comes up with flight available over the net four days that will take you to your requested weather conditions whether that be for skiing or sunbathing.