1. make sure you wash your model before painting to get all the mold release and finger oils off...
2. except for the silver undercoated model, use a primer.. MM makes one as well as Tamiya, but good old Krylon works just as good... primer helps the acrylic stay where it should, on the model...
3. what are you thinning with? sometimes that can make a difference...
4. over the silver you just have to be very careful with masking, the silver doesnt give the paint much to bite into... you could try using some very fine grit sandpaper and rough the area you are going to paint over first... the microscopic roughness gives the acrylic something to stick to as well.. Tamiya makes a finishing sandpaper that whould do just right to give it just enough for the paint adhesion without leaving gouges.
hope this helps... I know the frustration of seeing paint come off the model after all that work... primer was the main thing that stopped that happening for me...