This is one of my favorite things to do before a painting: A small 2″by 3″ graphite sketch.
There are many advantages to this: It’s low risk, it’s quick and dirty. Above all, it gives me almost instant feedback if the masses, shapes and values work. Do they relate to each other? What about lines? Since it is a very small sketch, it can easily be altered, erased and redone without major time invested.
I find that if my little sketch looks good, the final painting will have a better success rate. Maybe it’s because it boosts my confidence in a subtle way.
The sketch can also have another nice effect: it can tell you if it just isn’t worth painting! At least you have found out in the early stages..time to move on change your approach!