Variety of course...
that's why we had a butler, a pool boy, a gardner and a Candyman (er, I mean handy man but my name is Candy and I renamed him:)
No, not at all.
Common sense. Ensure that as a couple you remain having time together. People get busy with careers, children, attending their functions and the couple becomes more like roommates. Money is tight for many but finding someone to watch the kids is a must as dates must happen, at least once a week. One must compromise. One must do things with their spouse because they know their spouse likes doing those activities (both in and out of the bedroom). Of course the other partner needs to reciprocate too. Too many couples believe they are doing more in the relationship than the other one and they begin to resent this. Learn what little things your partner likes and do them from time to time. Keeping things fresh isn't really brain surgery, it's more about not taking your spouse for granted and then actually doing them. So many don't and expect that things will be OK. Well, a barn will deteriorate if it isn't kept up, painted from time to time etc... All things begin to deteriorate if not given attention and upkeep and so do relationships. People say well, we're married and I love him/her and they know it. But they get busy with life and allow things to deteriorate. How much does it really take to draw a bath, light some candles and tell your wife to go take a restful bath while you go keep the kids occupied. She'll love the half hour to herself. How many people actually DO this? Some yes, sadly nowhere near enough. If a man plays softball or church league basketball, his wife needs to be able to do things, play bunco, meet up with the ladies too. In the bedroom, communication must also take place. A partner must do things for their lover that they like even they don't like doing them. Not all the time, but from time to time. If there are things that are off limits, they need to be discussed so one isn't secretly hoping and waiting for them to happen. Don't always have sex the same way, the same time of day, in the same room. Spice it up. With the kids gone, I've known couples to go have sex in the car in the garage as they used to have sex in the car as teens. Why not when you're 32 yrs old?
Variety. Bringing new experiences into the bedroom. Being able to be open about what you want, and what you're curious about. It changes for sure. Sometimes it's the same old same, and of course some of the passion is going to subside, but those sessions that remind you of when you were dating still come around. And sometimes it gets even better.