The main problem with quite old decals or with these of poor quality that these decals like cracking.The old Mastercraft decals are the best example.Simply if you put them to water you had colour puzzles instead of decals.The only way to avoid that is to cover the decal sheets with a thin layer of a gloss (matt) cote before these decals will get water. Another problem is the one which was mentioned by Otftch and others.How to settle the decals on a model and avoid their shrivelling?The Ed's solution seems to be good.But we should remember that the model details can be very fragile.What is more ,there cannot be enough space for squeezing the decal over the details, especially on models in smaller scales.Therefore, I think that the best solution of the problem is to use ,the mentioned early, special liquids for decals or buying other decal sheets.
Of course there is possibility of making our own decals or painting these markings with patterns (masks) on a model directly.