What is the healthiest way to become a vegetarian?

The ADA suggests the following tips for following a vegetarian diet:

• Choose whole-grain products (e.g. whole wheat bread, brown rice, or whole-grain cereals instead of refined or white grains.
• Eat a wide variety of foods.
• If you eat dairy products, choose non-fat or low-fat varieties.
• Use eggs in moderation.
• Limit intake of sweets and high fat foods.
• Use a regular source of vitamin B-12, and if sunlight exposure is limited, of vitamin D..
• Many vegetarian foods can be found in any grocery store. Specialty food stores may carry some of the more uncommon items, as well as many vegetarian convenience foods.

(info from brown.edu)