If you make a larger sketch be sure not to invest too much time or make it too precious. You want a working document that you can change without reservation. Remember to start with generalities and work toward details. I like to stop short of including too many details as I find that finishing the sketch exhausts my enthusiasm and I want to preserve as much spontaneity as possible.
As you practice this aspect of painting you will find that getting a useful sketch will be a great help in achieving the success you desire.
Brad Teare © 2009