Suggestions/Advise on storing portland cement ??

Here's my dilemma: I'm very very busy and those bags of cement are just too much for me. I can, on occasion, get a friend to carry one of those bags from my truck to my garden shed. But what happened last year was the friend helped (I think in March/April) and carried the bag to my garden shed. This shed is wood frame base with tin covering. When I finally got the chance to try to work on a project, my cement had hardened in the bag. I know it was fine when purchased by punching the bag and I didn't open the bag. I thought my shed would be plenty dry for storage, but perhaps the riding garden mower housed there along with garden tools (rakes, shovel, hoe, etc) caused too much moisture. I think I was going to try a project in September...so about 6 months.

Does portland cement do this a lot? How long can you store it? Are there any secrets to keep it from hardening before its time?

My shed is about 75' from my drive way on some uneven turf. I did buy a dolly to move around a few other heavier things, but I don't know if I even want to attempt to try to remove a bag from the high bed on my truck to the dolly. And then there's always that 17 miles to the HD, Lowe's etc to purchase some and I really don't do those on spur of the moment when I feel like trying a project.

Any advise or suggestions? Thanks.