I start out with 2-6" blocks under each skid, as the season goes by I usually end up putting another small block on top of the original. Water will flow out of the holes at times because of the weight of the shack, causing the ice to build up under the shack, stay on top of that so water never gets to the level of your skids. I am always able to retrieve the blocks, as in the spring the sun will melt the ice around the block, a bump with a sledge hammer will get them out when the timing is right.
I would suggest while you are building the shack to incorporate a removable drop down jack for each corner, just to be used when jacking the shack up. I have done this with both of my shacks, it is very convenient and safer than a handyman jack. I cut off part of the handle of the jack and weld a big nut to the shaft, then use my cordless drill to turn the jack, very fast, can have the shack blocked up in 5 minutes with little effort.