Plenty of water plus:

L-arginine (1,500 mg per day)
Zinc (100 mg per day)
B-complex (take as directed(
Vitamin C (500-1,000 mg per day)
And apparently Soy Lecithin (never heard of this until yesterday. lol It's a large component of semen and apparently increases volume, but not thickness)
Plenty of edging (pro porn stars stay horny and are brought to near-orgasm many times by "fluffers" all day long and shoot the cumshot at the end of the day, and this makes them shoot a lot harder)

Necessary disclaimer: If you have any disease, it's very important to consult with a licensed physician before starting any vitamin or supplemental regimen. This not intended to diagnose, treat, prevent or cure any disease.