1. Dress appropriately, make sure that you are wearing neutral clothing and no whites.
2. Then have a clear vision of what you want with your painting before you start.
3. Make sure that your palette and canvas are out of the sun. Keep in mind how the sunlight will move.
4. Make note of your light source, light and dark sides. Is there reflected light? Strong or diffused light?
5. Where is your lightest light and your darkest dark. Every other value falls in between.
6. Color intensity. Where is the brightest, strongest color? Every color relates to that.
7. What will be your focal point? How will you support that idea? How will you make the focal point more important.
8. What are the edges like? Hard edges are usually reserved for the focal point.
9. Look at the colors. Is there harmony?
10. Is the light warm or cool?
11. Will you have to be extra careful about drawing or perspective?
12. What can be eliminated in the scene?
13. How will you handle brushwork? Should some areas be handled differently?
These are some thoughts to consider before you even put the brush to canvas. Your best paintings will be when you take some time to evaluate the scene and think it through.