1) BUY AN OYSTER CARD IMMEDIATELY. A day pass in London now is like $17 (which is absurd), but with an Oyster card it's only about $9. Oyster cards do cost $6-$7, but it pays for itself in a day. Your Oyster card will automatically cap at the daily rate, so there's no need to worry about getting a day pass everyday or buying a day pass and not utilizing it. Plus single fares are cheaper with an Oyster card too. If you don't want to wait until you arrive in London to grab your card, you can sign up for a card and have it shipped to you in advance. Just don't forget it at home... Like I did. For more details visit their site.
2) Don't take the tube everywhere. It's usually faster to just walk or take the bus. Plus it allows you to see more of the city.
3) Don't listen to other sites when they tell you to get a London Pass. They're paid to tell you to get one. The first time I went to London I got one. Then I realized that most of the places that I wanted to go were free. I ended up doing things I didn't even want to do just because I felt like I needed to get my money's worth.
[Free Museums: British Museum, National Gallery, Natural History Museum, National Portrait Gallery, Science Museum, Tate Modern, British Library, The Tate Britain, Victoria and Albert Museum (above), and more.]
4) The best pubs are hidden in alleys. The problem with this is I can never remember where they are... So talk to the locals. They'll point you in the right direction. Or just wander around and listen for the distant sounds of drunken laughter.
(Although I must admit that I love the Red Lion in Westminster. At night, the tourists thin out and the politicians come out. And let me tell you, those politicians know how to drink.)
5) Screw fish and chips, Indian food is king. Dishoom is the most popular Indian restaurant in London, but there are plenty of amazing places to go. Check out CN's list or just head to Brick Lane for a plethora of options (all within walking distance).
6) You have to pay for mixers at the bar. When I first went to London and ordered a gin and tonic, I didn't expect to get a pour of gin and a separate small bottle of tonic. Then I was even more confused when I got charged for said bottle of tonic. I quickly learned to use a bottle of tonic over the course of 3 drinks. (My G&T ratios are like 7:3. This tip won't work for all.)
7) What the firk is a firkin?? I went to London before craft beer took over the world, so I had no idea what a firkin was. I just thought Brits liked their beer warm and flat. Nope. Firkin is just cask-conditioned beer that has to be manually pumped and has less carbonation. Don't be afraid, kids. Get yourself a "real ale."
8) With that said, all of the beer in the UK is chilled. Stop spreading rumors.
9) Pubs close at, or before, midnight. If you want to keep drinking, you'll have to head to a club.
10) Know your voltage. Just because you have an adapter doesn't mean you can plug anything into it. I may have caught my adapter on fire with a hairdryer... Don't be me.
11) I'll never fully understand cricket, so I stopped trying.
(I just realized that half of my list is alcohol related. Figures.)