The #1 question I'm asked as a fitter is, "How can I make my shoes last longer?". There is no magic answer. However, the better they are taken care of, the more likely they are to maintain their integrity and take longer to break down.
The best way to take care of your pointe shoes is to keep them away from their nemeses, moisture and heat. Exposing the box to these can soften the paste and other materials, causing them to break down faster. That is why it is important to let your shoes dry out after each use. The easiest way to do this is to remove all padding from the box and lay the shoes side by side in a mesh bag for 24 hours. Avoid leaving them in hot cars, beside water bottles, etc.
There are many theories as to how to keep a paste shoe from breaking down quickly, but the simplest and safest way is to keep them dry and at room temperature.