Abstinence and healthy lifestyle.
Apparently it takes 64 days to produce a fully matured batch of sperm.
Semen is predominantly made from water and contains other minerals including zinc and fructose.
Hormone levels will also dictate your load size and quality.
Eat a good balanced diet of nutritious whole foods, get some exercise, stay hydrated, get good sleep and abstain.
This should serve you well outside of any health problems.