You can extend the life of rosin by limiting its exposure to air. This may sound ridiculous, but I keep rosin in a little plastic container from the deli with a spoonful of water. I leave the container on top of my TV cable box, which is always warm. It's like a miniature greenhouse; the rosin doesn't dry out.One of the biggest challenges of using rosin is knowing when to throw it away. As bassrosin.com points out, it's rare that an entire cake of rosin gets used before its freshness has passed.
Sounds anal retentive, but it works.