That's a really tough question that I think varies from couple to couple. (We're going to use a singular him here, but this advice applies across the gender/relationship spectrum.) Here's one really important thing to remember about relationships that become "stale": A lot of the time, staleness is the result of a neurochemical rection. When you get really close to someone (or become comfortable enough to fart around him), your brain actually produces hormones that make you feel content, comfortable and relaxed in your relationship rather than aroused, excited or crazy in love. 

It's the opposite when you meet someone new. Your brain produces chemicals that activate sexual arousal and make you feel super fucking excited. But that does not, in any way, mean the loss of that wild excitement and arousal is wrong, bad or the end of your relationship. People always make the mistake of thinking that feeling of arousal and excitement should last, but it's not going to in that same way, thanks to how hormones change the way you feel about a person over time. 

There are things you can do to keep the relationship, as you said, "hot and steamy." you need to have an open conversation about what turns you on, which might seem weird because you've been fucking forever. If you don't voice what you wnt, you're not going to get it. The hardest part about doing that is you've got something to lose. You don't wnt to frek your partner out or mke him think you've chnged or he doesn't know you as well as he thought he did. 

If you approach it right, you won't have to worry about that. Let's say you really want to put peanut butter in your boyfriend's butt and lick it out. Post it like you're still super turned on by your partner. Say something like "We've been together for three years but you still drive me fucking crazy, and it turns me on." Stroke his ego a little. Then, sexily, tell him there's something you want to try, that you can't stop thinking about doing with him and only him. 

"I want to lick peanut butter out of your butt," you must say. Even if he doesn't go for it (who wouldn't? Peanut butts are sooo normal), you've at least made yourself vulnerable emotionally, thereby increasing the intimacy between you, while at the same time letting him know you're still the wild, crazy sexual beast you were when you two met, nd you're still down to try new things. If that doesn't make things hot and steamy, then…fuck.