You can work the stem by squeezing it between your thumb and forefinger until you hear a light crackle/crispy sound. Don't do it so much that the stem ends up shaped like a football " () " your stem should be " O " shaped. The plant will lean over it the stem are worked out enough but it shouldn't look weird or awkward. If you do this enough times a day it will bulk your stem up significantly. Just work the stem all over. When you hear the cellulose crack you can move on the next part of the plant. Eventually you can work a plant enough to be able to bend stems that were previously hard and brittle.
I do a little more but I can't explain it well enough to recommend it...
Basically it take the more flexible parts of the stem, closer to the top of the plant (warning: do no do this to the stiff parts of the plant or you WILL break it in half) between my middle finger and forefinger and carefully bend the plant slowly untilI can get it to bend 90 degrees or more, as kind of a bend and stretch exercise.
Then I grab just above and just below the part of the stem want to work with both hands, and twist my fingers in opposite directions to break and stretch the cellulose inside the plants.
If you do this everyday, for 20 minutes a day, you can get stronger stems without having to use additives or spend any money.