If this is your first trip to London, stay for at least four full days in order to hit all the classic sights like Buckingham Palace. But whether it's your first trip or your tenth, the key to visiting London is planning. The more you dig around, the more you'll realize how many things you can do in the city -- and everybody will want to see different sights and attractions. Before you go, decide which which tours or sights are a "must" for your group.
Transportation is notoriously expensive in London. Take the tube or a taxi at least once, for the experience and the picture. Locals travel by bus or by foot. The bus map can be a little confusing, but it is reliable and much cheaper. You'll be an expert by the end of your trip, which will give you a better excuse to come back again!
They aren't kidding when they say "foggy Londontown." There's always a threat of rain, which is something that bonds the English together in solidarity. Even in the summer, you should carry an umbrella or light jacket. One idea? Plan your visit around a classic London event (like the Oxford Cambridge Boat Race down the Thames, or seeing the Christmas lights at Harrod's).