Because I work transparently (a personal choice), I'm really big on doing as much planning as I can before I start any watercolor painting.  This includes thumbnails, often a combination of a line drawing and a 3-value study.  The 3-value study is only set aside if my photo reference (or working from life) is a perfect value study in itself. 

Taking time to generate a plan expedites your painting, letting you concentrate on working with the paint as it flows onto your paper rather than having to problem solve as you go.

If your reference isn't providing a perfect value thumbnail like the study of the lighthouse below right, take the time to do a value study.  A value diagram of your composition allows you to spend your time concentrating on what the watercolor paint and bleeds are doing on your paper instead of getting bogged down with (compositional) problem solving while dealing with wet paint. 

The scene of the Corinthian in Marblehead, MA was painted on location at sunrise.  This is a photo of the scene later in the day. Working from life (outdoors) teaches you to deal with ever changing light...to work fast or set it up so you can return to the same location at the same time of day.  Of course, that works only if Mother Nature is being co-operative.

Taking time to do studies of your reference material, even if you're working plein-aire (outdoors).  This allows you to familarize yourself with the structure, design your composition and/or experiment with combinations of colors, washes and bleeds before tackling your actual painting (might save you some $$ too).  The study on the right is a charcoal drawing.  I like doing studies in charcoal as it gives me a more painterly look which I can relate to actual brush strokes.  I also get a good idea of where to melt detail into larger more ambigious areas.  This helps cement the composition together (into a unified look) instead of having bits and pieces of details bouncing all over your paper.

Try new palettes or limiting your palette (as I did with the painting of the "Quinn" house in the center.  I used only burnt sienna and ultramarine blue.  Different brands of watercolor will give you different qualities of color, especially with burnt sienna. In this painting I used Winsor-Newton's burnt sienna as it will go to a bright orange if watered down, giving you a sense of warmth and brillance that other brands won't achieve.