This is a no-brainer if you're a frequent rail user! Even if you don't plan on using the rail in the next few weeks, if you download the Trainline app via Amazon's appstore (Android) you get a FREE £8 credit when you use the app to purchase a rail ticket.
Where I live, some short journeys can be made for £1.10 so if you want to pack your phone with apps, it's still worth buying the cheapest ticket you can find just to get the credit.
Here's what you need to do:
Download the Amazon Appstore App here
Make sure you've gone into your settings and given your phone permission to download from an unknown source
Now your app is downloaded, find The Trainline App and download that
Next, buy yourself a cheap train ticket!
This is open to the first 50,000 applicants and finishes on January 20th. You need to use your Appstore credit by February 21st.
Big props to jonnithomas for finding this :)