How do I make a decal look old?

You can make a template,spray it on and then weather it.If you use a decal just weather it with a wash,and rub it with dry pigments to scuff it up

One word. Dry Transfer...well make it two words. Dry transfers are much easier to weather than decals.

After applying your decals you can overcoat it with a suitable clear coat (flat or gloss depending on personal preference and/or desired weathering technique) so that you have a uniform overall surface for weathering.

Having the same clear coat on the decals as on the rest of the model means that any weathering which you apply will have the same overall adhesion.

I airbrush on a very thin (almost transparent) coat of the base color under the decal.    This makes it look very faded.

Thanks guys for all the good information! Only problem is that I'm a new modeler so I'm going to have to research the techniques you guys suggested. lol But thanks loads; again.