1. Buses run all night, but be careful not to drunkenly fall asleep on the 25 or you'll find yourself in Ilford at 4 a.m.
2. The tube is your best friend. The map actually makes sense, each line has a different color and name... I could wax poetic about the tube forever. But it closes at 12:30, so plan accordingly.
3. However, avoid rush hour if at all possible, unless you enjoy being crammed into a car like a sardine and standing with your nose in some strange person's armpit in a hundred degrees. And if you MUST travel during rush hour, avoid the Central line. And Bank station. And Liverpool Street station. And... ugh, just avoid rush hour, okay?
4. Big Ben never gets any less beautiful.
5. Out exploring in the city for a day? It's entirely possible to not spend a dime (other than for food and transportation).
6. For as much as I love the tube, it's a ridiculously walkable city. Do yourself a favor and wander.
7. ...Because there are MAPS! EVERYWHERE! Pretty sure they were put in for the Olympics, but it makes the city laughably easy to get around. If you're in a main area of the city, there's a map posted every couple of blocks. It's impossible to get lost (and that's coming from me, born without a sense of direction).
8. What Londoners call "rain" isn't actually rain. It's more of a gray, drizzly mist that you're not really sure you need an umbrella for. But you WILL most likely be wearing long pants through most of the summer.
9. Fish and chips. High tea. Just do it.
10. Appreciate the fact that this is the greatest city in the world.