How can I keep Magic foils flat?
Sleeving cards helps keep them flat. Additionally, there are "perfect" or "pro" fit sleeves, which fit cards tightly and can be inserted inside other sleeves. The stiffness of double-sleeving will help a card maintain its shape, as well as protecting expensive cards from rough shuffling or when they are migrating between sleeves.

This, in combination with a deckbox that compresses the cards a little (most of the ultra pro plastic boxes will fit 75 double-sleeved cards, just about) will keep your foils from bending too much. In general, the best advice is to keep them in a deck and keep that deck packed tightly.