So I googled this question and not surprisingly found this:
How to Properly Store Ammo

Is it a good or bad idea to store ammo in ziplock bags? Assume that its already in a cool dry place, will the plastic bag provide extra protection from moisture?

For long term storage having it in food saver bags is much better

Short term kinda pointless

Plastic bags CAN be good for storage but remember a couple things;

1) When you seal it, you are also sealing IN any moisture (humidity, sweat from your hands, etc.) in with it. So you should probably add a dessicant as well.

2) Cardboard and paper always hold a certain amount of moisture. If you expose a plastic bag containing a cardboard box to high temps or direct sunlight you will often see water condense on the inside of the bag. Just something to think about when you consider storage locations and environment.

3) Plastic bags are thin and easily cut, torn or compromised. Also, most plastic "baggie-type" bags are permeable to some degree, so over the long term, moisture can get in but not readily get out. Again, this is more pronounced with extreme temperature changes.  

Ammo can with seal in good shape, desicant inside, good for about forever.

Is there a desiccant 101 thread anywhere?  I finally got around to ammo-canning a bunch of ammo in .30 cans.  Do I use little bags, loose shake?  Where to buy?  Feeling a bit dumb/ignorant right now...  

I have been looking for the small green plastic retangular shaped dessicant containers for awhile (silver metal ones seem to be more available).  I believe they can be recharged in the microwave and have a window to see when the dessicant changes color.  I have yet to find a source wher they can be purchased in smaller quantities.

This is what I use:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=261444966613

I draw a bit of a vacuum on these, straw and a fish tank pump does well, with desiccant in the can and the primers/bullets sealed it will last longer than I will.
So far have all the rifle and shotty ammo done this way.

Pistol I havent decided exactly how I will deal with that.
Lot of work to seal a few k of that stuff.

Anyway get the good bags, dont cheap out, and pull a vacuum so any air leaks in will be dry from the can.
Works well if you dont overload the bag.

Somewhere in the world they are laughing at this thread.  It's ammo not bread; cool, dark , dry and consistent. I dropped a new non sealed WOLF 7.62x39 round in a jar of water for a week, dried it off and fired it ; went bang ,rusty poly case and all.

I agree with the guy above me.

I live in the Northeast where my home has some pretty big swings in both heat and humidity but then again I am not living on the fringes of the Everglades either. Toss the cardboard box on a shelf in the gunroom and I would more or less think it is good for a good long while.

I have never had any problems with my stuff , some of which is 40 years old. I have shot up some other stuff I ended up that belonged to members of my gun club that was stored in a similar manor that I estimate to be 50 or 60 years old..

Back about 1999 I helped a friend shoot up some ammo that he had bought as a 12 year old from bannermans in New York back in the 1930s
It was 38rimfire . he had bought a case of it and it had been stored in a nasty old barn that had a bad roof for at least 20 years before we shot it.
It had a few duds and some squibs but according to the nasty paper package it was made in 1896 .

Long term storage can mean different thing to different folks but I get a chuckle out of the folks who spend much time and effort to wrap and store ammo that they are going to shoot in three or four years.

Bunch of guys in my club a couple of years back did a bulk buy of some surplus stuff then they got in a big argument about the best way to repackage and store it because they didn't like the spam cans The cans looked like they had gone around the world in the bilge of some tramp steamer but when they opened up a couple the insides were near perfect , why mess with something so perfect?

My brother shot his first elk with a Nosler Partition out of a batch that my dad reloaded in 1979.
It had been stored in a conventional ammo box in Southeast Alaska, one of the wettest places on earth.  I have fired more, they are all GTG.  I think I'll keep the rest...