Everybody has their own "best" way to care for reeds, but there are a couple things that everyone agrees on.
NEVER leave your reed on your mouthpiece. It will get unbelievably gross and not last very long. Always return your reed to a case (the case it came in or one of a million different kinds that you can buy).
Use several reeds in rotation, the more the better. If you play one reed at a time until it gives out, not only will it not last as long, but you won't have to panic if a reed breaks right before a concert.
I usually have only 1 reed per sax at a time, for the main reason that reeds go softer, denser, and moldy depending on how much you use them. Dont leave a reed on ur sax, after a week it wont be possible to play it because there isn't any moisture.
I recommend using a reed guard or something like that, which is just a case, which doesnt let moisture escape.
If you ever break your reed b4 a concert or the like, just spend half a hour playing with it, or bend your reed, or if your really desperate, use some fine water resistant sanding paper.