Have you ever read "How to Make Love to the Same Person for the Rest of Your Life" by Dagmar O'Connor? It is an oldie but it has very good advice. This is from the back of the book:
Here are the techniques, exercises, and settings which can keep the adventure and seduction (and even a delicious touch of illicitness) in your relationship--to make committed sex the best sex there is.
Why home can be the unsexiest place in the world--and what to do about it.
How to deal with "I'm in the mood for sex, why aren't you?"
How "sexing out" instead of dining out can be the answer.
How to "Unmommy" your wife and "Undaddy your husband.
Why the "sexual seesaw" is the most dangerous marital game.
How to think like a teenager--even if you're middle aged.
How to turn your spouse into your Dream Lover.
How to resist temptation and bring that sexual tingling back home.
You can get this book used at half.com or Amazon for about $7.