9. Smoking . We all know smoking is bad for you, but if you've never quit given them up, keep in mind that they will likely reduce your semen volume as well. One study looked at fertile men and grouped them into non, mild, medium and heavy smokers. What is interesting is that hormone levels (testosterone, LH and FSH) as well sperm motility (mobility) and density did not differ significantly among the groups. The one characteristic that did differ was semen volume, which decreased with increasing levels of smoking.