There was a time when our old bass player took certain tablets before going on stage. He still does to a degree. They tend to be of the sanatogen variety these days though, according to his mate ...

I have started learning / relearning songs using the iPad rather than sheets of paper. I must look at On song but for now just put them as PDF in iBooks. They are then also available as a quick reminder on the iPhone before singing them when out and about.