I've always got away with just working the pistons in-and-out (my oldest brakes are 12 years old and counting). Preferably, fit some old worn-out pads to protect the pistons, put a knife blade into the gap and push the pistons back into the caliper. Squeeze lever to bring the pistons out again. Repeat until pistons move freely. A drop of hydraulic fluid into the gap can help lubricate the seal, but it's probably best to shift any grit first - soapy water and a brush should be fine for that.
Just be careful not to allow them to come out all the way (the pads should help avoid this). If you get a sudden flood of hydraulic fluid, you've allowed the seal to come out too far. If this happens, just make sure everything is clean, ease the piston back in, and then re-fill and bleed the system.
