I was 95% vegetarian for about 5 years and found it worked pretty well, even without specific rules.
In general, I ate meat at major family holiday gatherings, when I was traveling and there were no filling vegetarian options, occasionally when there was particularly ethical meat available, and when the meat in question was clearly headed to the trash can if I didn’t eat it. I think overall I ate meat about once a month, but I didn’t keep close track, and the times I ate meat were pretty clustered, so it’s hard to estimate. I certainly felt that I was achieving my goals in being vegetarian.
One thing that helped me not decide that every friend’s birthday was a special occasion was that I just told everyone around me that I was vegetarian. Since upsetting people’s expectations makes me uncomfortable, I would only very rarely eat meat in social settings.
It changed for medical reasons, so unrelated to how I felt the policy was working for me in terms of balancing temptation with my reasons for doing it. I’d like to go back to it, or something like it, but I don’t know how to do it without spending a lot more energy thinking about food than I want to right now.