zinc
saw palmetto
protein
maca
creatine
glutamine
the last two are important because creatine helps with keeping fluids in cells, therefore more hydration .... glutamine is used by cells that are prone to fast turnover, such as those in your digestive tract.
also try doing kegels ... they strengthen the muscles that help you to ejaculate and therefore the stronger they are the more oomph you're going to get ...
the other, absolutely guarenteed way to get gallons of the stuff is to use a cattle prod up the poop shoot ... you do need to be unconscious because of the potential pain, but it is used very effectively by most zoos to collect large samples of sperm from animals.
mind you, that defeats the purpose ....