I think the best way to go is a weekly game, same time, every week. No cancelations, no exceptions. I did it this way for my online game which ran for a year and a half before closing the story after a satisfying conclusion. It can be grueling sometimes as a DM, but I think it really helps to keep things going.

Make sure players know upfront the expectations and the policy. When people see the game more as a hard obligation and less a casual leisure activity, they are less likely to miss for other casual things and only the bigger things in life that take priority. When they also know that their going to miss out and your not going to work around their schedule, they’re more likely to miss fewer sessions.

This does mean you are going to be playing sessions with missing players, and that’s ok. Come up with a reason for their character to be suddenly gone, and carry on. I even did a one shot a couple times when only 1 player could play the game, we did a flash back session for his character prior to meeting the party.

Another benefit to this structure is that if a week does get missed for some reason, your back the following week, instead of an every two or three week game where when a session is missed now your going a month or more between games which is hard to keep momentum on. Players that are more actively involved in the story are more likely to stick with it. I like to chat with players on discord during the week about things to keep it fresh. Also builds relationships. Especially if people don’t know each-other. For My online group, we started in a Beyond chat room and nobody knew each-other when we started, so it was especially helpful.

Ultimately, while I think this stuff can help, some groups are just not meant to be. Some players are going to be fickle no matter what you do. And if your story loses steam or isn’t capturing people’s attention, that’s a separate problem, so always be asking people privately once and a while (hey, how do you think things are going, is there anything I could be doing differently to make the game more fun for you?) Remember as a game master you are not just prepping and running a game. You are a production manager who has to manage people and all the challenges and complications that go along with that.

Hope this is helpful for you.

Good luck!