Being a vegan, I get a lot of questions and comments when people find out about my lifestyle. Their questions range from "What's your favourite food?" to "Do you feel healthier now that you're a vegan?", and their comments range from "I would be a vegan, but... ['I couldn't give up meat', 'My parents wouldn't let me', 'Carbs make you fat', etc.]" to an "Oh" followed by an awkward silence (which is by far the most ominous, in my opinion!).
The truth is, if you're the first vegan (or vegetarian) a person meets, they're likely to judge veganism as a whole based on the assumptions they make about you. It's been proven by social scientists that one negative experience with a member of a demographic group can jade people's opinions about that entire demographic. When you present yourself to others as a vegan when you first meet them, you are becoming the example of veganism to these people. This is why it's so important to be a good example of veganism to people who you meet-- it could affect their views of  veganism for possibly even years to come.
Being a good example is especially important when it comes to health. Many omnivores believe that vegans are generally unhealthy people-- which isn't true, of course. However, if you look unhealthy, people may assume that this is 100% due to your vegan diet. (Even if you have some unrelated health problem. I acknowledge that this is totally unfair.)
Make sure to eat a healthy, balanced vegan diet and get enough nutrients. "Oreo Cookie" Vegans, Obsessive Vegans, and people who don't bother to take fortified foods and/or supplements especially have to be careful of this. Exercise if you can, and get holistic treatment for any health issues that are bothering you. Not only will this make you look healthier to outsiders, it will also make you feel healthier, so you can help more animals and generally live a good, fulfilling life.
If you have the cold or the flu, I recommend not choosing to wear your animal rights garb that day. If you do, people may look at you and say, "Oh, look at his/her shirt: 'Vegan'. But he/she looks so unhealthy!" They won't realize that you just have the flu.
If you are suffering from complex health problems, I'm certainly not blaming you for not looking healthy as a vegan-- that would be cruel! If they are feeling up to it, people with poor health can still be animal rights advocates, although it might cause meat-eaters to be slightly prejudiced. If you can improve your health, then try to.
A good way to make sure you stay healthy on a vegan diet is to read vegan nutrition books (such as Vegan for Life or Becoming Vegan) or consult a dietician or nutritionist. You can also go to www.veganhealth.org for free, scienifically-backed information if you just need a bit of quick info.
It never hurts to inform people of the following:
Veganism can be very healthy if you do it properly, and many, many of us vegans are healthy! :)
And remember to set a good example; try to be as healthy as possible. Wear your "Vegan" logo shirt when you go running at the track, shopping at the grocery store, and walking the dog. People will look at you and say, "Wow! That person looks so healthy! And they're a vegan? Wow! Maybe I should go vegan, too!"
Have a great week! :)