I had very good luck with the BritRail flex pass, which allows you do use it for non-consecutive trips/days over a certain time period. This was a few years ago, but at the time, they were supposed to punch or stamp it on the train, and for whatever reason, most of the conductors didn't--they just looked at them instead--and we were able to get quite a few extra trips. Even if we hadn't, it was nice to have some flexibility.
You can buy regular Oyster cards at any tube station, including the ones at Heathrow, but if you're taking Heathrow Express into London, rather than the Tube, it might help to have the Oyster ahead of time. I think that if you want the visitor Oyster card, you have to buy them ahead of time and have them sent to you. (I think the only difference is that, with the visitor cards, you don't get the deposit back when you leave.)
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