Mix it up, as the kids say
Maybe the problem stems from too much vanilla-ness in your lives. This can be solved by a couple of ground rules: what if Tuesday nights feature pressure-free missionary, and Thursday is a free-for-all fuck fest where nothing’s off limits? Just spit-firing here, guys.
Try positions other than the normal ones you were taught in school, and consider surfaces in your house that haven't felt the gentle lick of naked flesh since 'Nam. Why not have sex on your counter every Thursday? That way, you can spend all of Friday disinfecting that very same counter and preparing a delicious home-cooked meal.
Don't fret rainchecks
No matter how much omnipotence you think you have, you just can't put a schedule on life. Maybe the big night has arrived, but you’re suffering from the worst bout of food poisoning you've ever had.
Don't just cancel! You must reschedule.
The point is to always tend to each other's needs, and acknowledge that you both want to do it with each other. It seems obvious, but the monotony of monogamy can make people forget how romance works.