Partially stolen from a comment I made over at TRT's place, and based on my extensive experiences in the place, i.e., about four trips, the last of which was in 1998. Presented as a public service announcement for Bob, who apparently might need it in her new foray into things both medical and educational.
Disclaimer: this will probably annoy people who actually live in London and know it several million times better than I do. Ah well, you get what you pays for, and in this case I'm dispensing advice for free.
So, without further ado, I give you:
The Knowledge (rev. 2.o)
1. Street numbers in London make perfect sense at all times. They start at 1 (or another number), and count up one side of the road. At some point (perhaps at the road's end), they turn around and come back. Occasionally numbers are skipped. Sometimes they count upwards on both sides, even on one and odd on the other, like in other cities. This happens just frequently enough to be annoying, but not so much that any Londoner either realizes it, or will be able to help you to find things when it does.
2. The streets in London are laid out on a nicely geometrical grid system. Nicely geometrical, that is, as long as you accept that a) the Earth is curved, b) it's curved a lot in tiny areas scattered randomly throughout the city, c) the term "nicely" can be interpreted in many ways, and d) "geometrical" can refer to a number of different non-Euclidean states. Oh, and it also helps to accept that the first sentence in this paragraph is wrong. I blame Ralph McTell.
3. Finding addresses in London is easy, as the founders of the city and many subsequent generations of inhabitants, local politicians, urban planners and general smart-alecks have conveniently divided each street into several contiguous sections, each with a different name. If you can actually find the street you're after, there's barely any of it to look in for the address you want, so it's a bit like cutting a deck of cards twenty or thirty times and then guessing at which stack contains the Ace of Spades. You're bound to find it pretty quickly. Unfortunately, you have to go through the other 29 bits of street with the wrong name first, but it would be churlish to complain about that, surely?
4. To counteract the effect described in point three (i.e., one street, many names), London is also helpfully filled with many streets each having the same name. These are differentiated one from the other with arcane codes like "WC11 left 5 right 3", "E17N22BINGO!" and "SE-11YouSankMyBattleship!". They are also quite usefully separated by several miles of intervening neighbourhood, so if you're at the wrong Bank Street, it becomes pretty easy to tell after you've been wandering around it for a bit.
5. Another useful aspect of street naming is that very cleverly, the same name is often used for different streets (closes, crescents, avenues, places, squares and so forth) in close proximity to one another. This makes it easy to get into the right general area, and damn near impossible to actually find the address you're looking for. You're liable to end up sleeping on the grand piano in some minor royalty's town home, when what you were really looking for is an uncomfortable cot in the hallway of the Oozing Wound Clinic at the local hospital. But that's ok, Londoners are used to this kind of thing, and tolerate unexpected guests reasonably well.
6. The last point, and this is an important one, is that the public transit system is excellent. Which means that when there is a bomb scare as the theatres let out (frequently), and absolutely all of the taxicabs are occupied by people just a bit quicker on the uptake than you are (inevitably), there is still a wide and bewildering variety of buses available, almost all of which don't go exactly where you'd like them to, but nevertheless will get you tantalizingly close before rounding Nelson's Column or some other picturesque piece of architectural paraphernalia and zooming off in just the wrong direction, about five seconds after you realize that it's gone just a little bit too far away from your destination to be useful. This public transit system is the envy of the world, and has been emulated in such meccas of excellent transit experience as Delhi, Manila, and the South Pacific island of Pingelap.
7. There is little else that you need to know, except that all of the Tube lines go to roughly the same places, eventually, as long as you're willing to spend an average of four and a half hours, change trains six times, go up twelve escalators and down fourteen, and take at least one taxi. Speaking of which, the blokes driving those oh-so-quaint black London cabs (some of which are coloured maroon, confusingly) actually do know where everything is, so maybe you're better off taking the taxi first, as long as you have 300 pounds or so in spare change to pay the fare.
There, wasn't that helpful? In a future installment: getting to and from London by rail - so many options, so little likelihood of getting the right one. Heathrow airport, I shall leave for an entire episode all its own. There will be pain and anguish and foul language and moaning and gnashing of teeth, oh yes.