Stop trying.It may be old-fashioned advice but it's true. When you try too hard you can "psych" your body out. Other things to take into consideration...
How stressed are you? Too much stress shuts down certain chemicals in the body that are required to work together to get pregnant.
How's your diet? A woman's body will not accept pregnancy if your diet is out of whack. Too many proteins? Not enough proteins? Decent variety of fruits and vegetables on a daily basis? Too much salt? Too much or too little sugar? Do you snack through out the day? Too much snacking can throw the vitamin and mineral balance out. Large amounts of caffeine? All of these things can off set the required balance. A body needs certain levels of functioning vitamins, minerals, proteins, iron, and carbs before a pregnancy will stick, this is so the body knows that you can support and maintain a life to term.
If you are in doubt, try taking pre-natal vitamins now before you get pregnant in order to bring about the proper balance. You can get them over the counter at most drug stores. Don't worry they won't hurt you. It's like taking a multvitamin but with a more complete structure for pregnancy.
On his side, if he wears tighty whiteys or any underwear that keeps his testicles too close to his body, the heat can kill off sperm before it has a chance to relocate; especially if he burns hot when he sleeps. Request boxers for while and when hanging out at home have him go commando under those sweat pants to allow more air circulation and cooling.