In class, I've been talking about and demonstrating the usefulness of thumbnail sketches. They allow you to quickly work out different compositions before diving in to your painting. Often the first, most obvious ideas may not be the most interesting compositions - and drawing a series of thumbnail sketches allows you to make that discovery before you even begin your painting.
YouTube is a great resource for learning about art - you can watch demonstrations of paintings in progress and instructional videos created by artists for other artists. Here are a couple of videos where you can watch thumbnail sketches being created:
"Thumbnail Sketches," a video posted by AnnelieArt, is a 48-second example of playing around with lines and moving different compositional elements, just to see what you come up with.
"How to make a thumbnail sketch for an oil painting" is a minute-and-a-half long demo from Expertvillage of creating a values sketch (values refers to light and dark).
Both are quick, good reminders of the possibilities thumbnail sketches allow us to see.