I invariably think about 2 studies in preparation for a painting: a composition sketch and a quick and rough value study.
A composition sketch serves to organize the picture space, simplify forms, look at what’s important and (more importantly) what’s not: Invariably, photos have too much detail and picking out what is important is best done before you paint. Also, things may not be where you want them, so you can easily move stuff around. You can also easily check out different scenarios – perhaps that holiday shot should be cropped!
Some artists include values (that is, lights and darks) as part of their one preliminary sketch, however it’s a good plan to do one separately. The trick is to force the picture space into 3 values: light, mid-tone and dark…and be rigorous, no in-between values! This promotes confidence to then block in values in your painting and adjust for detail afterwards. Any evident value problems, perhaps a light form next to a light backdrop are fixed when you go through this process. Keep it simple – it works best like this and I’m sure you’d rather be painting! The attached image gives the idea.
This website expands on the principle and makes some good sense: http://www.artistsnetwork.com/articles/art-demos-techniques/make-the-most-out-of-your-sketches
Happy preliminaries!
David