Drink more water or juice (liquid makes liquid, and what we adults tend to drink -- coffee, alcohol, or tea -- can dehydrate you a little), take your daily recommendation of zinc (the prostate needs zinc to function properly, as well as your reproductive system).
Since I began taking the proper amount of zinc daily my amount of semen has increased.
Other nutrients like lecithin can also help, depending on the person. I've read at least one anecdote where a guy swore that lecithin intake increased his semen load. With me it's hard to tell, but I take it anyway for other benefits.
Taking supplements you may not spurt like Peter North, but you may see some improved results, and with the proper amount of zinc your reproductive system will probably be happier overall.
Good luck.