Sketching like the masters

I have been looking back through a book on British watercolors so I can learn from the masters and improve my plein air sketching skills. Too often I get caught up in working too fast without a proper plan when I use a sketchbook. I want to correct that. I want to sketch more and use sketches for inspiration or components for larger paintings. I feel like I am running out of good reference photos, but if I would do more quick sketches, I would have a better feel for the scene anyway so things that might make a dud photo would make a good painting if I edited (through sketching) on the spot. Sounds good, now I need to put in the time. And get over the fear of wrecking sketchbooks with false starts.