Eat healthy. There is nothing specific to increase fertility, but a good healthy diet (including a vitamin with folate (folic acid) for her) can be beneficial....And give you the energy to keep trying! ;-) definitely avoid any smoking, and alcohol in excess.
Regular diet. Normal, regular diet is usually sufficient for great majority of couples. Almost anything in moderation is usually fine. Obviously, if one is overweight and/or has cardiovascular risk factors, then the proper diet would be prudent.