I spray a clear coat acrylic over even the gloss paint. It catches the light and works wonderfully! It also mimics what happens with metal as well, losing very little of it's shine as time goes on. If you want to shine it up, simply spray on another thin coat.
If you follow this guide you should be able to make it stay that way. Although it can be hit or miss for a gloss spray. Varnish would work but I've also heard that Future acrylic floor polish works as a great clear coat for metallic paints. I haven't tried it yet myself but intend on it in the near future.