Make your thumbnail sketch on paper larger than you would normally need.  Do not box in the sketch. Start in the middle of the paper and just sketch; include more than you think you will use. Include some suggestions of dark, light, and middle tones.
 
Now use a pair of cropping corners which can simply be cut from mat board, card stock, foamboard, or even cardboard. Slide them over your sketch while forming differing shapes of various sizes. These should include:
 
       #1 Crop = a landscape format
       #2 Crop = a portrait format
       #3 Crop = a square
       #4 Crop = very small (a close up view)
 
First cropping choices are seldom as good as later choices. Make sure each shape is tried in several sizes. Pick the best of the best. Using your cropping corners, now draw in the correct rectangle on your sketch. Go to work painting. You gave up being "ordinary" several steps back

 

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