If you think you might push the pistons in too far, then removing some of the brake fluid from the reservoir stops it getting messy. I've used a syringe before to try and reduce the chances of splashing the paint work etc with the brake fluid. And, you might as well buy some new brake fluid in case it needs topping up, unless you already have some, but keep it closed as its hydroscopic.
To increase the ease of getting the caliper / brake pads back on I've used a screwdriver between the disc and the metal bit of the brake pad and gently twisted it a small amount. Though sometimes wobbling the caliper just to get the pads onto the disc can work as has been said, hopefully your discs don't have a lip on them which can make the job that bit more problematical.