• Use a Lunch Bag - Whether you buy a re-usable bag or use a paper one, make sure it's opaque and seal it closed. It's not fail-safe, but people are far less likely to go rummaging through a closed bag for food than when the same food is in plain sight.
• Require Some Assembly - Swiping an already-made sandwich is one thing, but stealing all the fixings is another. Try keeping the components and ingredients for your lunch separate and then assembling on the spot.
• Buy in Bulk (or Hide It Well) - This is good advice in general, but anything that comes individually pre-packaged from the store is also just begging to be stolen: individual containers of yogurt, those small packages of baby carrots, cans of soda, and so on. If it's something like yogurt or carrots, keep a larger container at home and use a small reusable container to bring portions to work. If it's something like soda, bring one can every day instead of storing several in the fridge at once and keep it out of sight in your lunch bag.
What doesn't seem to work? Writing your name on your food or its container. We'd like to think that the co-worker we say hello to every day in the halls might not steal our food if they know it's ours, but unfortunately, this doesn't seem to be the case.
Alas, no method is truly guaranteed. What thief-deterring tips do you have?