Eat Sperm. Technically you'll have more.
Not that I'd subscribe to that, but, then again, I haven't a problem.
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Just a guess: Get the vasectomy reversed.
Another guess: Avoid fatty foods and cholesterol type stuff, things that'll clog the pipes. Celery and Pineapple have been said to help move things along. Also I've heard cranberries/juice is good for you. I don't think any foods will increase production, but, if there's problems finding the way out then drinking lots of fluids and avoiding salty stuff, etc, might help.
Link
Lastly, as said above, this would best be something discussed with your/a Doctor.