MicroScale "Liquid Decal Film" is fantastic stuff. If I even suspect a decal may crack, I'll coat it with the stuff first. You can brush on a fairly thick layer (be sure to use a brush that's absolutely clean) and it almost disappears when it dries. Also, try using warm water when you soak the decals, and let them soak for several minutes if necessary..especially if they're old.
Mr. Friedman is right..you really need a nice, smooth surface for decals (I've been using Future for years..it's much less caustic than laquer and doesn't yellow.) A solvent solution like "Micro Sol" or "Solvaset" can also help them settle over surface details. I also second the recommendation for
http://www.squadron.com/. They're probably the best "one stop shop" for modeling stuff..and have great prices and service, too.
As for Monogram decals, they've been notorious for their terrible quality. I don't know about the more recent kits (within the last ten years or so) but anything they produced from the early '80s through the mid '90s were unusable. They were extremely thick, and refused to lay down no matter how well the surface was prepped or what solvent was used.
Cheers!
Steve