• It may be spelt Leicester Square but it's pronounced Lecester square. Also hoe-bun (Holborn).
• In London, always look *BOTH* ways. Just because the sign says it's a one way street, doesn't mean anyone is going to adhere to it.
• Don't leave your bag on the floor. Ever. Sit it on your lap, or on the table in front of you.
• Keep your wallet in an inside pocket of your coat, zipped up. Keep your Oyster card (get one!) similarly safe somewhere but not _in_ your wallet. If one gets nicked you still have the other to get back to your hotel with.
• Please stand on the right, walk down the left of the escalator.
• If the tube carriage is very full make sure you move all the way down inside. Don't worry, if you ask nicely people will let you get off at your stop (but not if you've pissed them off by hogging all the space)
• Wait for the little green man before crossing the road (if you're waiting at a crossing). People who know the roads will often leg it across because they know that the traffic lights are about to change but you won't know that and you don't want to be splattered across a bus' windscreen. I've had instances where I've legged it across the road and unwitting tourists have just followed assuming it's safe. Don't be a lemming.
• If you have friends that you're going to meet, get a shitty pay as you go mobile for the trip. It makes life so much easier.
• Go to Borough Market. It's open Fridays and Saturdays. Get there about 11ish, just walk around and try some of the sampler food there and enjoy the smell of proper food. Grab a Veal burger if you can be bothered to queue. They'll probably be serving mulled wine at the beer stall as well this time of year.
• 'The Clink' museum is nearby as well which I think is a little better than the London Dungeon and Borough Market is also where you'd want to start your walk along the southbank. Just follow the crowds of people. You'll go past (this is not exhaustive) The Golden Hinde replica boat thingy, The Clink, the Oxo tower (good but expensive restaurant and bar), Tate Modern (well worth it and free), St Paul's is just over the Millennium Bridge, the IBM building, the National Theatre, British Film Institue, over the river at Waterloo bridge is Somerset House (inside is the Courtauld Institute of Art, if it isn't cheap then it is free. I was a student at King's next door so had free access), between the QEII exhibition centre and the London Eye there are lots of stupid living statue people which some tourists seem to love. They used to just get in my way. Opposite the Eye is the old GLA building. You can buy your tickets for the eye there but a little further down, past all the shitty museumy things, there is the Namco arcade. Think there's also a Manga Museum somewhere near there as well.
• This might sound callous but don't give homeless people money. They need help, not just change, and your money would be better spent helping charity's who help them. Shelter or Emmaus for example or the Big Issue. Big Issue paper sellers are the one exception to the rule. It not only is a good publication but it also helps the seller out with getting back on their feet. Just make sure they are wearing their ID card and aren't hard selling. I personally like the guy who hangs out outside Somerset house.
• People talk about the cold but the rain is worse. It's been pissing it down here for weeks now so bring a waterproof coat and some boots (not Wellingtons, you won't be able to walk for long in them). Umbrellas will get snatched out of your hand by the wind and will potentially maim people like me who are trying to get passed you on the street.
• If you're lost or trying to have a look around don't just stop in the middle of the street. There is probably someone walking behind you (possilbly me) that you just stopped in front of and slowed down. This is London not Milan so no plodding around aimlessly. Either march like the rest of us or get out the way (please).
• If you're an Apple fan you can go to the Apple store on Regent Street. They do tutorial talks etc. sometimes if you like that sort of thing. Otherwise you can check out the Hamley's window display while you're there. Otherwise, most the shops around there (and Oxford street) are the typical high street stores.
• If you're going Greenwich, take the clipper. Not only can you see the sights on the way but I think it is a bit faster and generally more civilised.
• Only use the black hackney taxis or ring up and get a private hire taxi. Either way they should be licensed (private hire cars should have a TFL green square thing stuck to the windscreen). Don't use an unlicensed cab.
• I enjoy just walking around most places rather than getting the tube just a few stops. The central parts of London are often a lot closer than you think. If in doubt, ask someone for directions.
• As this is Slashdot I'm going to assume you'd enjoy Forbidden Planet. The largest one is on Shaftesbury Avenue. It smells a bit funny but I spent a good portion of a day there the first time I went.
• Then try going round the corner to Charing Cross road, go to Foyles' bookshop (have a look round if you want) and then go to the first floor and the Jazz cafe there. It's often pretty busy but you should be able to squeeze in somewhere. The coffee is some of the best in London (in my opinion) and they have free - if a little flaky - wireless. I spend a lot of my free time here.

It may be a little overwhelming at first but it really is a great city. I love it here and have no desire to leave any time soon. Have fun!