This site: http://germannn.tumblr.com/post/86891058047/selbst-vs-selber claims that "selbst and selber are entirely interchangeable; the only difference is that selbst tends to be used in formal speech and writing, whereas selber tends to be used in everyday speech".
I think its like do and make in english. If its something that you have to do with your hands, Or it's towards an objects, You would use make in english and use machen in german. If it isn't done by your hands and it's not an object, You use do in english and tun in german.
At least that's the way I see it.
Its like having something to DO in a sunday morning.
Or MAKING a cake.
Cake = machen. Abstract action = tun.
Hope someone can help me clarify this or correct me in case I'm wrong.