Most major airlines belong to one of three alliances. A primary consumer benefit of the alliance system is that you can redeem miles for travel on any member of the same alliance. Here's an overview of what this means in the context of Membership Rewards:

Transfer to BA (British Airways) for travel on any OneWorld carrier, including AA (and also on Alaska Airlines, which is not a OneWorld member but has a separate partnership with BA.)

Transfer to DL (Delta) or Flying Blue (Air France, KLM) for travel on any SkyTeam carrier.

Transfer to AC (Air Canada Aeroplan), CO (Continental) or NH (ANA) for travel on any Star Alliance carrier including UA (United) and US (US Airways).

Choosing which program to transfer to in an alliance requires some research. ANA's redemption schedule is based on distance, while most US-based airlines have a zone based redemption schedule. You also need to take into account that Continental and United now have shared ownership and the airlines will merge their operations and programs. No one knows whether the resulting program will look more like Continental's program or more like United's.

You also need to understand exactly what each program means by expiration. In many programs the entire balance is preserved provided you have any activity every 12 or 18 months - and this is easy to accomplish without travelling. In other programs the oldest miles expire after NN months without regard to account activity. Whatever you decide, do not transfer the points and then forget about them for years. You need to pay attention.