Hollywood star John Travolta has revealed the secret behind his successful 15 year marriage to actress Kelly Preston - they write a complex series of lists. The couple use the lists to make sure they make enough time for each other and sex - among other things. Travolta, 53, explains, "We make lists of what we need and want from each other and check them regularly, "We list everything from how much sex to what kind of food we want. "It's so we can find out what the other needs and that we're going in the right direction. It doesn't sound romantic but it works for us."