GG How Do You Keep Your Male Performers Hard?

GG I was wondering what your routine, supplements, vitamins, exercises, etc. That you have your male performers do/suggest to keep that rock hard on in your scenes for them to be able to preform for hours and not have problems keeping it up? Obviously they have hot ladies in front of them and im sure are on Viagra but still It's got to be hard for these guys shooting 3-4 scenes a week. How do they do it? What is their/your secret? Fluffers, Penis injections? Very curious 
