A pair of pliers, set so that the jaws are open ~½ inch, work well for me (for SRAM or KMC links). It helps to relieve chain tension @ the link first with a piece of coathanger (or large paper clip) bent into a "bracket" shape. Then grasp the link w/pliers, the jaws gripping the opposite side plates (i.e. diagonally) and open the link while squeezing the side plates with you fingers.
Install by diagonally gripping the opposite way, if it is a tight link.

Remove & Install a bicycle chain - YouTube

..skip to 1:00 minutes.