The non-vegan food items in my diet include:
Morningstar Farms meat analogues (which contain egg whites)
and really, that's about it. I don't eat much that has gelatin, honey or whey in it, and I despise eggs. I think the most important things I can do to prepare for such an experiment are to research vegan protein sources, stock up on margarine and soy milk, and do a lot of advance meal planning. It might not happen real soon.
And it's not very likely to go past 30 days, either. Even armed with a copy of The Uncheese Cookbook, I don't think I could go without cheese forever.
In light of recent discussions I've been involved in, I'd like to add that I take steps to reduce the harm I do to other creatures because it feels good to me. Someone posted an opinion on a Metafilter thread that people who attempt to eat a "cruelty free diet" are hypocrites who are mearly "trying to cover up the stench of the dead bodies piled under every object in your house." (Meaning that living karma-free in an industrialized society like ours is impossible for anyone other than possibly subsistence farmers. )
Maybe that's true. But I don't approach this with as lofty a goal as trying to build a world free of suffering. I'm only trying to build a Rachel as free of suffering, and as free of the pain of being the cause of suffering, as possible.
Plus, It might be fun.