Marketed as 1/76, I believe these stencils would do nicely for 1/72 vehicles as well.

As a pictures speaks a thousands words, I'll let the scan below do the talking.

Using the stencils is fairly straightforward, at least on flat surfaces. You place the stencil where the marking needs to be, fix it in place, mask off the surroundings and spray the color you want the marking to be. Take care to have the stencil as close as possible to the surface as possible and to keep your airbrush perpendicular to it.
While not as easy to use as decals, you have the advantage that the result has an infinitesimal thickness and that your marking can be any color you like.