I am a vegan.
My advice:
1. In addition to eschewing all animal products, also avoid all processed foods and hydroganated oils and trans fats. My rule is that anything other than a fruit, veggie or bean must have 4 grams of fiber or more per serving (a serving being between 100 and 250 calories). Avoid all white flour and processed crap.
2. Pay attention to your protein intake. I try to get 60 - 80 grams of protein a day (at least during triathlon training season). To get enough protein here's what I do: (a) eat high protein/high fiber cereal with soymilk in the a.m. (Kashi go lean or wild oats performance) this gets me about 20 grams of protein, (b) snack is an apple - about 2 grams, (c) lunch is bean soup or salad with beans on it, etc about 12 - 15 grams of protein, (d) snack is a vegan boca burger (15 grams) or a bag of edamame (20 grams), (e) dinner is a salad with beans (12 - 15grams), a stir fry with tofu (20 grams), or whole wheat pasta with veggies and beans (25 grams) (f) nighttime snack is fruit (2-5 grams).