Everything you do in your life, and some more! Your age, lifestyle, diet, whether you have any reproductive difficulties like endometriosis... Your weight, whether you get enough vitamins and minerals, and the amount of stress you are under. And, many other things besides those. How do you deal with these factors? Some, you can influence you can stop smoking, lose weight, take prenatal supplements, and eat well. Your age and any chronic conditions, you can't change. Besides living in the most healthy way you can attain, tracking your menstrual cycle and making sure you catch your ovulation (with the help of ovulation tests!) significantly. And, don't forget about your partner!
While he is not the one who will get pregnant and go through nine months of both hard work and joy, he does matter! Men who are fit and healthy tend to have higher sperm counts, and a better quality as well. Men who have too much sex (every day or more than once a day) see a reduction in the number of sperm in their semen. And guess what? Men who have too little sex (waiting for a week or more between ejaculations) have the same reaction. One other interesting thing to note is that having intercourse first thing in the morning gives you the best odds of getting pregnant of course, during ovulation or in the days beforehand because a man's sperm count is highest at that time.