I'm a big fan of using lunch containers that can be easily washed and used another day in another lunch. Sandwiches are in heavy rotation in Miss Mads' lunch, so all of these methods of sandwich packing see regular use in our house.
1 - Plastic Sandwich Case
This case was given to me in a swap a number of years ago, but I've seen similar ones in Target and supermarkets. Other than the brand name being gradually erased from the top due to repeated washings, it's held up well.
The case is deep, so you can put other foods in there aside from the sandwich.
2 - Bento Sandwich Cases
Sandwich cases I've gotten through import stores are a little different than ones I've seen in local stores - they have vents!
Some have additional compartments - the pink one above has a lower level to hold sides - so they would require a band or belt to hold them together (unless you like to live dangerously).
My favorite part is that they collapse down when empty; easier to carry home, easier to store.
3 - Fabric Sandwich Cases
These are the cases that get used most often for Miss Mads' lunch. It's only been more recently that I've started trusting her with some of the more "fragile" pieces I've acquired over the years.
These cases are more than just cotton - most of the ones we have are made by Lunchskins, which have a velcro close and are made of a fabric originally used for pastry bags. They're dishwasher safe and pretty durable. Some ours have been in use for about 4 years and are still kicking. I've found them on clearance at various points at Target, which is nice, since our supply has been depleted from bags wandering away. (I'm not affiliated with Lunchskins or compensated in anyway, just had a good experience with the product)
We have one from Skip Hop as well, but it doesn't get used as much. It's not dishwasher safe, because of the printing method, I guess, and who has time for that?!