1. Take a break.
If stargazing isn't fun anymore, go ahead and give it up. That's right... quit.
Don't think about astronomy, don't read about astronomy, and keep your telescope safely covered. If it's too cold, stay inside and don't feel guilty about it. But mark a date on your calendar when you will start again. It could be a few weeks from now, or a few months, or simply when winter's over. But take a break until then and let your interest regenerate.
2. Pace yourself.
Years ago, when my joints were younger, I studied martial arts. One of my instructors, a wizened Japanese judo master, watched many students come and go over his long career. The new students were too keen and promised to attend class four or five times a week.
His advice to them? Don't.
"You'll lose interest if you come more than twice a week", he told them. "It's part of Western culture to overdo it. You start something new, burn yourself out, then quit and move onto the next thing. Pace yourself and go slowly at first, enjoy the process of learning, and you will stay interested much longer."
3. Shorten Your Observing Sessions.
Don't wear yourself down with hours-long observing sessions. At least not every time. Just go out for 30-60 minutes, especially if you're looking at brighter objects that don't require much dark adaptation. If you have a scope that requires 20 minutes to set up, consider getting a small "grab and go" telescope that's ready to go in a flash. Shorter sessions also remove the need to find a big block of time to observe.
4. Leave the Telescope Inside.
Try leaving your scope inside and observe with your naked eye or binoculars for 20-30 minutes.
Bring along a star map and learn the names of two or three new stars or an obscure constellation each time you go out. Get to know the fainter stars in each constellation and deepen your familiarily with the sky. Practice your averted vision to see fainter objects.
You'll become a better observer and develop an intimacy and personal bond with the night sky without relying on complex and time-consuming equipment.
5. Set goals.
You've seen all the easy objects: the moon and planets, M13, M57, the Andromeda Galaxy and Orion Nebula, the eta Carina Nebula, and so on.
Now what? Is that all there is?
Are you kidding me? There are more sights to see in 20 pages of Burnham's Celestial Handbook that you could hope to see in five years.
But to maintain your resolve to see more challenging and rewarding celestial objects, you need to set goals for yourself. Measurable, achievable goals. For example, you might set a goal to see all the open star clusters in Cassiopeia brighter than 8th magnitude. Or observe eight new double stars this month. Or see and sketch all the Caldwell objects visible from your latitude this year.
There are endless possibilities depending on your interest. But goals will help you move forward and stay keen, even if you're a casual observer.
6. Use your imagination.
Sure it's nice to track down a challenging galaxy, or glimpse fine detail in an emission nebula with your new OIII filter. But if you find yourself getting stale, step back from the eyepiece and think in grander terms about what you see.
The Orion Nebula, for example, isn't just an abstract grey blob in your field of view. It's the building blocks and fuel of hundreds of stars and thousand of planets which may one day harbor grand civilizations that, in a few billion years, might produce an astronomer like you who will look back with his telescope on our own modest star as it turns into a planetary nebula and devours the Earth and other inner planets.
There are many future stories and much science in that humble grey mist in your eyepiece. Let your mind soar.
7. Seek Inspiration.
When your interest flags and you can't muster the enthusiasm for stargazing, look for inspiration in the words of those who have gone before. Three fine books come to mind. "Seeing In The Dark" by Timothy Ferris, "Starlight Nights" by Leslie Peltier, and perhaps "The Stargazing Year" by Charles Calia. These books will rekindle your enthusiasm and remind you why you got into astronomy in the first place.