1. Stay in hostel dorms. (Or get a partner to split a double room with... Stupid accommodation conspiracy that allows couple to travel more cheaply.... :( )
2. Catch buses between cities. (Depending on the time difference and the price difference! It may not be worth losing a day, if a train takes half the time for only a few pounds more.)
3. Book flights that leave at a sensible hour. You may think you're saving money by getting that cheap early flight... Until you have to catch a taxi at 4am. Expensive and exhausting, it'll cost more and it's hard to enjoy travel in a sleep-deprived state!
4. Book hostels that come with breakfast. Make secret sandwiches for lunch.
5. Find cafes / restaurants that have lunch specials for around five pounds. Chinese and Indian places are often good for this - cheap and filling!
6. Supermarkets will often have a three pound sandwich / snack / drink combo which will suffice for dinner if you're desperate. Also - reduced for quick sale meals are handy if your hostel has a kitchen.
7. Walk everywhere - until your feet end up covered in blisters.
8. Buy an Oyster Card, and pray that there's enough on it to magically get you past the next tube station barrier.
9. Visit all the libraries, galleries and museums. I spent more than half the day at the Victoria and Albert Museum today, and could have easily stayed for a few more hours. I'm off to the Tate and the British Library tomorrow.