Thanks to all for the responses and suggestions. Seems like there's no special trick or best practice, just whatever works. My tanks are plain steel so I'll try the Sharpie method first since that's cheap. I already bought a set of 3" stencil cards from Staples that I used for the flag so I'll try it on the tanks. Nice and cheap. If it fades too quickly, I'll upgrade to paint using the same stencils and see what happens.
Now to find my first keepers...
Now to find my first keepers...