I've had my Aeron chair for a while and realized that I had to readjust the arms every few weeks - they would just slowly lower over time. I got tired of that and came up with a fix: 3/8 inch wooden dowels from any hardware or crafts store. Just cut two dowels of equal length (I cut mine to the maximum length I could - 4 1/4 inches). Then shave just a tiny bit off of each side so they will fit in the part of the chair under the arms (see pic). Then push the arms down onto the dowels and tighten. They can't go down any more and the problem is solved!
You may want to spray paint yours, but I'm happy with mine as-is.