Here's a bit of practical origami for folding a little handle for a take-out bag. I reverse-engineered this from a neat trick done by a waiter at a Chinese restaurant.
Step 1: Fold #1: Right Triangle
This first fold is easy. Fold the upper portion so that you have a 45 degree fold. The image shows the fold for making a left-handed handle. (Mirror-image it for a right handed handle.)
Step 2: Fold #2: Handle Slope
Take the point of the right triangle, and fold it down. What you want is to have the length of the edge above the horizontal crease to be approximately equal to the length of the edge below the horizontal crease. The reason for this is that the part hanging under the horizontal crease will be tucked up under the handle, and the tip of the over-hang will rest exactly under the point of the handle when you're done, for a secure fold that's easy to hold.
Step 3: Fold #3: Tucking the Over-hang
Tuck the overhanging part under the handle, and give the fold a good crease. Voila! You now have a handle.
If folded properly, the handle should be fairly secure, and will permit you to hang the bag on hooks, on your fingers, etc. Now, you don't have to clutch your take-out bag like a burlap sack.