In a relationship, sex , at first is great. Then, as life becomes more complicated, it becomes commonplace with much interference with obligations; work, night school, bills, kids etc, so couples tend to try to squeeze it in when they can, and usually in as little time as possible. Sex, then becomes mechanical and boring. Because men and women have such different timing mechanisms, it's a wonder, at this point, if it works at all! Also, men are also visually stimulated so the cute, shapely girl they fell in love with, slowly changes as time goes on, and simply isn't quite so attractive. They forget that they are growing older too.
The only saving grace is the friendship between you two built up, gets you by these trying times. If you ask a man who his best friend is, and it isn't you, something is not right. Quiet talking is so important.
One of the biggest things you can do sex wise is to try different ways of having sex....have variety. For sex to be good, couples need to spend time with foreplay...it is the major difference between ho-hum sex and an explosive experience.
Each experience should be better than the last. Find out what he likes. Be sure to use lots of lube...it makes a world of difference.( see my other posts about this)
Stimulate his senses..fullfill his fantasy. Smells can be alluring, too. Go shopping with him, and let him smell and pick out a perfume that turns him on. How important is this? I went with a girl many years ago who chewed spearmint gum. Every time I smell spearmint today, I think of her.(40 years later!)You could try a spearmint smell. Pick out a pleasant after shave for him. Sex in the dark isn't a totally good idea: you can use a very dim, pleasant light during sex so he can be visually stimulated too. Set aside this time and make this time very special...you want him to think no one could ever do it better. You want to appeal to as many of the senses as you can. Try and come back here..I want to know if I helped.