Several people have noted that you can extend the shelf life of your yeast
by freezing it; this is true, with certain reservations. Every time you
take a package of frozen yeast out of the freezer, moisture will condense
on it, which can shorten its usable life. For this reason, when I get a
large package of yeast, I follow this procedure: Get two containers, one
for the bulk of the yeast, and a smaller one for enough yeast to get you
through, say, a month's worth of baking. Put that smaller amount in the
smaller container, and keep that in the refrigerator. The larger container
can be kept in the freezer. When you've used up the yeast in the smaller
container, refill it from the larger one and return each to its proper
place. This way, your bulk container gets much less exposure to the
moisture which can damage it!
I got this idea from Alton Brown, on Food TV's "Good Eats". It's a good
system, and works well.
Susannah