This topic has been discussed in depth on prior occasions.
I think the most helpful is this thread:
http://www.flyertalk.com/forum/showthread.php?t=1451
However, these also are worth a look:
http://www.flyertalk.com/forum/showthread.php?t=3760
http://www.flyertalk.com/forum/showthread.php?t=6144
(Some of the reflexive links won't work, since
FlyerTalk has revised the archiving, but the information in the posts themselves may be valuable.)