I answered this about a week ago, right here.
ASU in Tempe, AZ has uncovered the activators for tanning, and you can buy the stuff online at places like tanreasearch.com. This isn't the junk you'll get at a tanning shop, but, the actual hormone that causes your skin to turn dark. It works in an amazing way, and was developed for albino skin and for folks with high risk of skin cancer.
Folks in the art of looking good for a photo shoot / show do the following.
1. Tan in the sun.
2. Tan at the tanning place.
3. Spray it on at the tanning place.
4. Take tanning meds.
5. Spray paint the paint on, at show time, and paint the abs in. It's just like painting a house.
6. Take a vasodialator.
7. Take dirueritc to remove excess water, "dry up" and "tighten up."
8. Take insulin to drive the remaining fluid into the muscles.
9. On top of all that folks put cooking spray (PAM, or stuff like it) to bring up the highlights.
It's all science, no different than tuning any other machine. Of course, you have to be an exceptional athlete in exceptional condition to pull it all together when the camera shows up.