how do you make a paint job shiny?

Just the same way they paint real 1:1 cars to get the depth and shine. It depends on the surface, the paint, the application, equipment and mostly the person skill at doing it all.

Be sure that your thinner is compatible with your paint. Some thinners will de-gloss a glossy paint. You want one that will allow the paint to retain its leveling and gloss properties once applied.

You may want to start with a light cover coat of the color being applied, again as a hot rod painter would, rub out the paint before continuing onto the next. Hand rubbed paint jobs always seem to be the best.

Instead of a clear, you might want to apply car wax.

Let the paint cure for several days before applying anything over the paint!

A clear gloss overcoat. Many people like Future floor polish...it gives a super-shiny and reflective finish, but it does take a little practice to apply it well.  Future can also be mixed with acrylic paints (I've tried it with Tamiya) for a glossy finish. After drying (24 hours generally), it can then be buffed with a microfiber cloth for that extra high sheen. If Future isn't available, there are other name-brand clear gloss variants out there, such as Tamiya clear or Floquil's overcoats.

Floquil: http://www.testors.com/category/137406/Clears