I agree completely with siamesegeek; saving up and a good diet can certainly improve your load but genetic is the main factor. Either you shoot big loads or you don't, eating tons of rice and tuna won't turn you into Peter North if you have an average load to begin with.
And I love Yuna Shiina too btw (Waap made my day when they got her for their Dream Shower serie, hope you have seen this one)