I've always struggled with separating electric plugs and sometimes have broken the clips off.

Finally getting frustrated enough to take the time to make a tool, I came up with the following simple but reasonably effective little tool. It's made out of a stiff piece of wire, bent as shown, with the hooked end sharpened. Coat hanger wire would work, as well as anything reasonably stiff. I made two of them to disconnect the injectors and other plugs that are stable enough to use two hooks on.

For plugs in a wiring harness that you need to hold and pull apart, I lift the tab on one side and pull it slightly apart on that side by angling the plug, then use the tool to lift the other clip, and then pull it apart.