There's nothing I hate more than wasting money unnecessarily. Like when you discover you've paid twice as much as you needed to for something, not because it was greener, or better made, or anything like that. And if you travel on public transport in London without an Oyster card, you'll be doing exactly that.
But not to worry! The solution? An Oyster card. Nothing to do with shellfish, it's just a bit of plastic. Everyone visiting London should really get themselves an Oyster card, to save a fortune on the Tube, buses and Docklands Light Railway. What is an Oyster card? I hear you ask. In a nutshell, it's a pay-as-you-go top uppable swipe card for refundable £3 deposit, just add coins or even money with your credit card at all tube stations and many newsagents and small shops near tube stations - and it allows you to travel on all of the above transport, just by swiping the card as you get on, and also as you get off (Tube only).