Good health and... Keeping good health, well hydrated, and reduce the frequency of ejaculation can increase some volume when it happens, but other than that there isn't much else you can do...Currently there is no approved treatment/medication available for this purpose.... Good luck..
Time & Hydration. Amounts will vary from individual to individual. Typically, the more time between ejaculations, the larger the amount. The amount of stimulation prior to ejaculation may also affect the amount. Additionally, being healthy and well hydrated will enable your body to produce it's maximum amount.
If your T is low. If your testosterone is low, then you can be treated with HCG or clomid (clomiphene). Some people have even better success on tamoxifen. Hcg stimulates the testicles directly. Clomid (clomiphene) and tamoxifen block the brains ability to "see estrogen". Since men need some estrogen, and since men don't have ovaries, the brain asks the testicles to make more testosterone (which the body can turn into estrogen later).