I recommend using gloss varnish to apply "layers" to a miniature.
1) Paint your tank as you normally would.
2) Put gloss varnish over the area you want to decal.
3) After drying, put your decal on the gloss area.
// because the area has a gloss finish the paint job under the finish will be maintained, and the decal will slide around smoothly so you can place it where you want to without destroying it. If you don't like the look of the decal, just remove it and put another one on. The gloss varnish will protect your paint job.
4) If you're satisfied, matte finish it and you're done.
Note: You should always use a type of varnish seal to create layers on your miniatures. This way if you mess up you can just go back to the previous layer without destroying all your work.
You can also use varnish to apply rust effects to your tank. For example:
1) Dry brush orange and brown over the entire tank to create rust effects.
2) Gloss varnish the entire piece.
3) Paint tank as you normally would.
4) Apply water to an area you want to apply a rust effect to.
5) Rub on the area with a wooden tooth pick.
This will remove the paint and create realistic 3D chipped paint look on the tank.
Some people use hair spray, but gloss varnish will do the exact same thing.