Decals have always been a problem for me, but after getting back into the hobby after 30+ years, I discovered that what little dexterity I have left isn't good enough for trimming around all the decals that come with model kits now (I remember when we were lucky to get half a dozen for the whole model).
Then I discovered the Squizzers scissors that Micromark sells. Yes, at $14 a pop they are expensive, but man! the aggravation they save you. These scissors are so precise, I can even cut around the individual ship's numbers on one of those very crowded Gold Medal Models decal sheets, or nip out one white star for a 1/700 scale Sherman tank. They have also been a Godsend for cutting off the extra aerial wires when I'm rigging airplanes.