Actually, I just did something similar.

I already keep daily logs about what I did during the day (e.g. if I had an exercise), so I added not eating meat to the list of recorded variables. This step was trivial. But now, whenever I am choosing a meal, I remember that I will have to put it on the record. And somehow this trivial inconvenience makes me choose the vegetarian meal more reliably than previously.

I started recording this two weeks ago. First week there was no difference, but the second week I ate vegetarian food for the whole week. I only broke the chain today, because we already had some pre-made soup containing meat at home.