If you ever need work done on your brakes, it’s best to avoid brake shops for a variety of reasons — not the least of which is that they’ll often use a Jesus wrench (the biggest Channel Locks in their box) to compress the pistons, a procedure capable of cracking cheap calipers, and almost guaranteed to mar the piston. The right way is a brake caliper compressor, a sort of high-powered caulk gun designed to slip into the pad recess. Lisle’s model 25750 is a perfect example, and pretty inexpensive at just over $32 before shipping from Amazon.
It’s also possible to make your own piston compressor. This version from Hinode is a good concept, but generally poorly made in the Far East. Some good steel and a few hours will produce a very nice home-made compressor, but it’ll be hard to beat the low asking price of Lisle’s version. It probably won’t fit high-performance calipers with more than three or four pistons, but those are scarce enough to be ignored unless you’re a Ferrari mechanic.