Another thing to do is have water everywhere! On the model, on your fingers and of course plenty of water on the decal. Sometimes when the decal is almost in place and you've rubbed away a lot of the water, DON'T try to slide it around at all with out applying some more water with your fingers and get it to kinda float again or it will tear. Another for instance, if you have  applied a decal that spans a door and a fender, slit the decal on the seam with a very sharp xacto blade and reposition the decal to leave the gap. You'll be much happier with the end result.

If a decal is old and even cracked, if you are careful you can coat them with a couple of coats of cystal clear Krylon laquer paint. You can even touch them up with colered laquer paints prior to that if need be. Even if the decals look like they may be the type that are really realy thin and will give you trouble use the clear laquer on them.  And one of the best rules of all is always trim your decalas close as possible to the image, using small sharp scissors. A lot of decals look great on the paper but when you apply them have a clear ring around them that looks awful.