In my experience of working on the same comic for six years with a lot of ups and downs, the best way to keep a comic successful is all about the creators behind it. Being flexible and forgiving on yourself, while also having a few friends to support your ideas helps so much.
If you aren't able to make dramatic story changes, alter art styles, and take in feedback (good or bad) from viewers and test readers, then it will be really hard to keep a comic successful as you will get stuck in a rut eventually. Being able to be patient and not push yourself too hard is also very important! Know your limits! I tried to do my comic almost alone every week during senior year of high school and freshman year of college and I couldn't handle it. I had to be forgiving on myself and give myself a break. This meant I could still always come back to my comic without starting to blame it for my fatigue.
Another important part is feedback of course! Having people interact with your comic can really inspire and encourage you a ton. That's the hardest thing to get though, since feedback is up to other people, not yourself. Having a healthy interaction with fans really keeps me motivated, so don't be afraid to post a comic even if it isn't perfect. Changes can always be made later, like with my comic. I changed the style 5 years later and came back to a new audience that is more responsive.
Being patient, forgiving, positive, and flexible can really help keep a comic going through good and bad times.
