Just read this really good honest article about sex in marriage.
If your sex life is flagging I highly reccommend reading this
Two years ago, my wife and I made it our new years resolution to have sex every day. The reason was simple. Utterly worn out by the arrival of our first child, we had pretty much stopped having sex. I was 33, she was 30, and our erotic life needed not so much a jump-start as electrotherapy.
Read the full article and the results of their sexperiment here http://www.timesonline.co.uk/tol/life_and_style/men/article5598881.ece