Oyster card

Buy it in London at a tube station.

I thought about getting it online, but I find the Oyster website confusing, and it may not accept foreign addresses.

If you are flying into Heathrow and want to take the Tube to central London, get the Oyster card first at the Transport for London Travel Information Centre where Terminals 1-2-3 meet. It's not hard to find from Terminal 4 but is a bit of a hike.

The cost is a £3 deposit (presumably refundable, if you're never coming back to London) plus however much you want to load onto it. I started with £20 for four days and didn't use it all. When you "tap out" the display shows your balance. If I had I could "top up" at any tube station.

One thing changed is that if you take the National Rail trains to points south of the Thames, the Oyster doesn't work, where the old multiple-zone Travelcard did. Inquiries to the TFL website about that went unanswered; I had to buy point-to-point tickets for travel to my host's home in the Kent suburbs (about £4 oneway/£6 return).

Just get it when you get to London. Its like getting a bus pass here in the States.

The Oyster card never expires so you can give it to family/friends when you get back.. or keep it for the next trip.

for our recent trip to London, we went to the offical London Underground website
http://www.tfl.gov.uk/modalpages/2625.aspx
and purchased a paper 7 day travel card instead of an oyster card. Worked just fine. The TFL sent us the card two weeks before our arrival. We got the card from the UK in 3 days.
We did have one ticket agent look at the card and state that these were no longer sold, but I told him the TFL sold it to me via Internet and sent it to me in the US by mail. He shrugged and let us through to the fare gate.

When we arrived at Heathrow in June, we stopped at the Underground Info desk as we walked to the Tube and bought 1 week cards right there for about 23 pounds. Very easy, and took care of all of our needs for the week.