Be careful with clear coating decals before adding them to the model. The coat(s) will help keep the decal together, but can make the decal thick, not blending into the finish and looking like a sticker.
With decals, warm water is your best friend. I always use warm water with a drop of liquid dishwashing soap in it. This helps break surface tension, and it slides better off the decal and onto the model.
Also, always wet the area the decal is going to go. People get into trouble pushing a decal onto a dry model and it sticks and breaks. Water, water water...Wet your fingers, wet your brush, wet the surface where you are going to set the decal down to slide it off onto the model. Did I mention water? After building for people for years, this has been my main lesson learned about decals. Patience and warm water solves 99% of all decal issues.
If the decals are just too old, brittle and simply vaporize in water, try the clear coat , or if they are dark enough (no white or real light areas) get a blank sheet of decal film. They come in letter size sheets, and go to the office supply store and have a color copy made.
Hope this helps.
aim9guy