there are lots of herbs that can help, as well as vitamins and minerals that can improve quality. as a general rule of thumb, the "male vitality" and "libido enhancement" section of the natural foods store will often have a great many of those herbs and whatnot in their formulas
A really good quality men's multivitamin will also have things like saw palmetto, green tea extract, and ginseng (to name a few) that can enhance sperm quality.
p.s. I had DH on a bunch of stuff for 4 months (because it took a while to conceive DS, probably due to male factors) prior to TTCing, and I got pregnant on our first try!