Advance tickets. And 16-25 railcards to save an additional 1/3.
Single all the way home to Tunbridge Wells next month (via London Euston - Virgin Trains + Tube) will cost me only £6.25.
You will need to buy in advance (at least a couple of weeks to get the very lowest prices) and be prepared to travel at off peak times. IIRC, when the tickets are launched 12 weeks in advance, there are a few £6.25 advance tickets for the peak hours, but these get snapped up quickly.