When my son started with kindergarten, I picked up a pen and a scrap of paper and listed all the food items he has allergic reactions to. This note has been posted on the kindergarten's kitchen bulletin board designated exactly just to indicate the children's dietary restrictions. I do the same when I leave him with his grandparents, or if he goes to parties and playdates for the first time.

Thanks to AllergicGirl, I came across AllergyCards.com, where you can indicate all food items you are allergic to, and print them in a handy calling-card-size format... for free! You can use the pre-made lists, or make your own one.

Why use an AllergyCard? You can use the French version when you travel, for example, so you won't have difficulties getting your dietary restrictions across. It is also especially useful for children, and lends them credibility, so that other people will know that they can't eat certain food items, not just because they don't like them, but because it can be really dangerous for them.