Tickets for the London Eye are typically £20.  You may want to avoid the queues by prebooking online (and saving 10%!) and collect tickets quickly from one of the collection machines; however, you may also want to make sure that it's a clear day before you decide to go as you don't get the full effect on a grey or cloudy day. 

However, you can purchase tickets online the same day, as long as it's a few hours before you plan on riding.  You can also avoid the queues by purchasing your tickets from external vendors, such as the tourist info offices or offices selling tickets for bus tours like the Original London Tour or Big Bus Company.  By purchasing a ticket in advance, you can skip the extra long queues, which is a great time-saver, especially on a busy day.  It is generally a good idea to visit before 11.00am or after 3.30pm, as these are off peak times. The flight is worth doing at least once if you want a bird's eye view of London.

There is an option to purchase a special ticket that allows one to see London from the London Eye during the day and once more during the night. 

If you used the rail at all, you can print off a two-for-one offer from the Virgin Rail website or check out National Rail 2for1 offers. There is no specific day you need to use it on, or cost associated with it, and it does save a significant amount of money.You just bring the print-out to the ticket office, along with yout rail ticket, and they discount your tickets.