look at your relationship wholistically to identify any causes/contributors
is she depressed?
does she enjoy life overall?
has your sex life staled out?
are there any ruts in your relationship?
are you making any of the classic mistakes ie bathroom door pen, changing in front of one another etc?
in my experience sex is a barometer of fun in your lives as well as in your relationship (unless youre prone to use it as a crutch) - as soon as something else in your life begins to suffer, so does the sex...so look at those first
either that or you can just slip some testosterone into her martini. after a while, youll dread it if she changes position in bed