I think it's important to have different pitches for different situations. You may encounter a situation where you have a chance to talk about your comic VERY quickly. Then a slogan or a short synopsis is helpful. If I had to describe or pitch my comic Big Sandy Gilmore that quickly, I would just say "Big Sandy Gilmore is geek humor, woodland style!"
If you're in a longer conversation, then the movie-plot synopsis is a great plan. Hit the high points and don't get bogged down in the details. Realize that the average joe doesn't care nearly as much about minutae of your comic as you do. Having a humorous newspaper-style strip, this is a little harder for me to do. It's not like my comic has much of a plot. I think I would just describe three to five of my main characters and the sorts of situations they might normally find themselves in.
It's also great if you're given a chance to talk about your comic in-depth, too. If you're up for an interview or writing an article, you need to be able to talk about the details of your comic to people who are interested in your process.
The key, in public speaking terms, is to have expendable and expandable content. Judge the the interest of the people you're talking to, and be able to add more or take out content of your pitch accordingly without sacrificing the important points. It's good to be intentional about your sales pitches. If necessary, practice in the mirror or with your family!
