This list is based on my experiences and those of others I've spoken to/read pieces by. It's basically a digest of what not to do if you want your friend/colleague/acquaintance to remain vegan - and what you can do instead if you find yourself in that situation.
-Nitpick. Politely pointing out things that you know for sure have honey/whey powder in is one thing. Grabbing and inspecting someone's packet of plain crisps (which are usually vegan) is another.
-Refuse to accept that people move at different paces. You might've gone from omni to vegan overnight (or at least that's how you remember it with the benefit of hindsight), but not everyone is going to. If someone tells you they're phasing animal products out, don't assume that's an excuse for never going vegan. At the end of the day, if someone takes a bit of time to become fully vegan then stays that way, it's better than making an overnight transition that they then have trouble sticking with - that's how ex-vegans are born!
-Get into a position where they're doing it for you. Not constructive. It's about the animals, then about themselves, not you - 'go vegan if you want to be my friend' just makes them question whether they want to be either!
-Berate them for not finding veganism easy. If you found it a complete doddle, fine, you're blessed with extreme willpower or luck or both. That's not the case for everyone. If they're asking you for help, bleating 'It's EASY' at them will not have the desired effect. Constructive suggestions for how to deal with problems such as
lunch at work, being
drunk around pizza and so on are however a good idea.
-Make veganism look harder than it is by introducing extraneous variables. A person does not have to be gluten-free, caffiene-free, raw, unsupplemented or anti-science* in order to be vegan. And a person certainly does not have to share your musical preferences or fashion sense in order to be vegan. Oh and by the way, wearing a shirt with a collar does not mean someone is an infiltrator.**
-Imply that the best way to help the animals is to kill yourself. (as a not-so-new vegan I was shocked to see that, but am a bit more able to deflect it. If it had been my first impression of veganism all those years ago, I would now be either dead or not a vegan. Neither would be great outcomes.)
*'Science' and 'vivisection' not being synonymous, but hopefully you knew that already...
**I never saw Mark Stone wearing such a garment during the years we were both in Nottingham. I, however, wore one on a regular basis because animal rights meetings were on the same day I taught class.