Trains in UK - Transport, Trips

http://www.nationalrail.co.uk/

It's simple - the earlier you book, the less you pay. You can do it up to 12 weeks in advance, but even the day before can save you a pretty penny in comparison to buying on the day. It's also an easy way to guarantee you get a seat.

Travelling off peak is a great way to get yourself better fares. You also have the added benefit of quieter carriages.