Let them bring the topic to you
I always say that I'm a vegan if eating out, meat or animal ethics are mentioned, and I always explain why if asked. Otherwise I never bring up the topic directly. However, I find that I get more than enough challenges and questions from omnivores to be constantly discussing and explaining why I'm a vegan. And I'm an introvert! I don't talk to that many people.
Be an example
If someone compliments me on my clear skin, apparent health or fitness or general exuberance, I say "I think it's the vegan diet."
Use cake
Feed people delicious things. Post pictures of the delicious things you are eating on social media. Give cake. In my experience this very often leads to a pleasant (because cake makes everything nice) discussion of why you are veg*n and how great it is (seriously, vegan cake is the best).
