If you've been in a relationship for as long a you can remember it can be hard to keep that spark.

You get comfy, you stop making an effort, you even start farting in each other's presence and worst of all the sex goes stale (That's what we're told anyway). But a new study has found the secret to a long and happy sex life - and it's pretty simple. If you still want to be knocking boots when you're claiming your pension - and beyond - the study says you just need to keep things fresh. The research, which looked at 38,747 couples aged 18 to 65 who were married or living together, found that adding variety to your sex life can keep the spark alive. The even better news is that mixing it up and trying new things can keep you as sexually satisfied as those glorious first six months of a relationship when you're literally at it around the clock. It is one of the largest studies ever to examine which factors can lead to a satisfying sex life long-term, reports The Chicago Tribune, and many of the couples had been together for more than 20 years. "If properly nurtured, passion can last for decades," the study says. "Nearly two-thirds of sexually satisfied respondents reported that their sex lives now were as passionate as in their early days together. "Over one-third of sexually satisfied men and women selected 'passionate' as the single best word (out of six) to describe their most recent sexual encounter." There was one reason above all that left these couples sexually satisfied - it was because they were up for trying new things. Reading sexual self help books and articles was a big factor in inspiring them to new things, according to lead author David Frederick, assistant professor in health psychology at Chapman University. "But what set sexually satisfied couples apart was that they actually tried some of the ideas," he said. The research found that satisfied men and women were more likely to have spiced things up with a new sexual position, acting out a fantasy or using a sex toy. It also revealed the couples who were more satisfied gave and received more oral sex, had more consistent orgasms, and and engaged in more frequent sex. No wonder your granddad's always whistling in the morning.