If you open a can of tomato paste and only use a small amount, here's a way to save the rest of the can so that it lasts for a long time. After opening the can and using what you are going to use, open the other end of the can so that there are now no ends on the can and then put the entire can in a plastic bag and put it in the freezer. Let the can of tomato paste stay in the freezer over night.

Once frozen, remove the can from the freezer and run it under hot water for a couple of seconds. Then push the frozen block of tomato paste out of the can. Cut the frozen block of tomato paste into 1" thick pieces (this is the size that works best for me, flavor your size to how much you typically use at one time). Then put the frozen pieces back into the plastic bag and back into the freezer. Your tomato paste will keep for months like this. Whenever you need some tomato paste, you can just go to the freezer and you'll have little blocks to get the amount you need.