Storing is easy, but making sure you dont contaminate the jar is just as important. They need to be ~
1) kept in a cool, dark, dry place. Light, heat and humidity affect them. I keep mine in a dresser drawer in my bedroom away from it. I keep them upside down, so some of lids are scratched but thats so I can see which colours they are. All my back-up pigments are in original boxes, in another drawer at the base of my dresser.
2) dont ever use them direct from the jar! This is so important if you know that you arent going to use the whole jar within a few years. Place a small amount from a sterile spoon into a sample jar and use your pigment from the sample jar only. Never dip brushes or fingers into the original pot!
3) Make sure they are sealed properly. If the lids arent on properly then no matter where you store them they will be affected.
I have had a few that turned yukky on me.... my very first Frost went a horrible yellow colour and clumped in the jar... I ditched it. My first Maroon kind of sweated and turned into a horrible icky mess inside it jar - I was distraught to have to ditch it, my second one is stashed away safely so it doesnt happen again!!
I have also heard of pigments in vials turning as well, changing colours and or becoming quite sticky so make sure if you have vials you keep them in a cool dark place as well