Remove it from the can and place in a platic or glass container, wrap well and keep in the refridgerator for up to 6 months. Treat like fresh nuts, they go rancid so store in a cool place. Freezing is good for nuts, not too sure about the paste.
I bought the pate pistache in france last january after attending a course at lenotre. The chef used it in one of the recipies and i thought that it would be nice to have it around just in case... but a few days ago i was looking at a book when the pate pistache was in one of the ingredients so i decided to give it a try and made "pistachier":
160gr' egg whites
100gr' melted butter
70gr' pistachio paste
mix all the dry ingredients together. add the pistachio paste and melted butter, then the whipped whites,fold gently. Pipe in a flexipan mould and bake at 220 c for 15 minutes. Remove from mould when cold.
This recipe was taken from "les Buffets sucres"- de L'ecole Lenotre....
You can purchase pistachio paste from most commercial bakery suppliers. I do love the taste! You use it much like any other flavorings such as almond or lemon. Either in baked items or frostings and fillings, creme brulee, cream anglaise etc...etc...