How can I temporarily level the ground for a pool?

My backyard consists of a driveway, a large cement slab, and a small area of gr, all of which are not level. I have a 3 1/2 foot pool that is 12 feet in diameter that I want to set up on the cement area, but like I said its not level, it slopes slightly to one side. I had the pool set up last year and by the end of the summer when I went to take it down the slight slope had shifted the water enough to the point that I was unable to get to the hole to drain the pool how it was supposed to be drained. I am looking for a way to level the ground slightly, but not permanently. Can someone please help?