the people that go vegan and stay vegan are the ones that have realized how disgusting meat/fish and dairy is and the fucking lies that the government/food and health administration tells us about nutrition....once you make the connection that animals are NOT food (humans are not supposed to be putting that shit in our mouths) you will never want to touch any animal product again...especially when you see how it goes from the factory farm to your fridge/freezer
you have to retrain yourself to be disgusted by meat and dairy...basically...
for most people, within maybe 3 or 4 weeks of being vegan your palette will completely adjust to your new way of eating and you will have no desire to eat meat ...these are the documentaries that that have veganized most meat/dairy eaters..
and like rig said being vegetarian is a first step but aim for veganism.....because dairy,cheese, eggs are shit on a plate (or in a glass).....worse than animal flesh in terms of the effect on your body...and watching these documentaries will make you see that