Flint pretty much covered the powder storage issue.
Like Flint I have a few cans of powders that are getting pretty long in the tooth but theyre still good.
The biggest concern to me when handling powder is keeping it identified. Knowing what you are working with at all times during the reloading session. I will make test loads for a particular round and bullet using 2 or 3 powders but I never have more than 1 type of powder on the bench at a time. I will make up whatever number of rounds with that powder than I seat the bullets, bag or box those rounds making sure to identify the powder, charge, primer, and bullet used. After those rounds have been put away I clean the bench, throwing away any powder spilt on the surface, zero the scale, and put any unused powder back in its can, seal it up tight and put it away. Than I get out the next powder and proceed again.
If you practice using only one type of component at a time you can eliminate most (not all) of the dangerous errors a person can make when reloading. That will go a long ways in keeping your gun in one piece, your fingers on hands, and not shaking the confidence of your fellow shooters setting next to you at the range.
:-)