[royr]

Have you ever leaned against or pulled on a towel bar and bent it?
A trick I have used many times is to put a wooden dowel inside the bar. The bars are usually round and quite thin. Take the bar to Lowe's or Home Depot to the millwork department and look for wooden dowels. There are oak and lighter weight, poplar, cheaper woods that work fine. Just try fitting dowels into the bar until you find the biggest diameter that goes in. The dowels usually come in 2 ft and 4 ft lengths and cost about $2-$3. Cut to fit, slide inside the bar, and install your towel bar as usual. It could also work for square bars if you can find square dowels.
Don't use the towel bar as a pull bar unless you have it anchored very securely.