On the pistons that are down in the hole, take some sand paper and get as much of the rust out of the cylinder as possible (no sense having more crud wedge between the piston and bore). I've had good luck with Kroil, but everybody has their favorite. If it's really bad you might consider vinigar or citric acid (both of which disolve rust) and let it sit for a week or so. Once you get it to turn a little rock the crank back and forth until you can get to rod caps and start removing pistons.