The Serenity Prayer has been around for about 100 years now. I find myself repeating it fairly often, especially now that I've got young children.
It gives you a blueprint of what kinds of things you can expect to be able to change and the limitations you'll face in the attempt.
* Which more in the context of the 12 step program the other person can be yourself. You will have thoughts, impulses, emotions, urges that you can't control but you can control your reaction to them.
Like it or not, but you __can__ control others. This is what advertising is based on, for example.
I use the prayer as a framework when I have to take a mental break and find the discernment between a situation I truly can't change and one I can influence (or, like you pointed out, a response you can control).