For this, I used empty gift wrap rolls that I chopped down to size with scissors. These boots are really tall, as they are over-the-knee length. For knee-length and calf-length boots, you can use an empty paper towel roll. With the holiday season coming up, I'm sure you'll have plenty of empty gift wrap rolls though! Once you're done bopping people over the head with them, turn them into boot inserts.
It's really very easy... You just stick them in your boots, and it's great. So much money saved on not having to buy special inserts for my boots to keep them upright!
Are you as bothered as I am by boots that don't stand at attention like a soldier?