Paintings with the most interest generally have a variety of values. Often when we paint we apply color - lots of color! - and think we have a beautiful painting. And it might be so. But to be effective the colors should not only be different in hue or intensity but also in value.
Take a picture of some of your paintings - include some that work and some that don't. Then change the picture to a black and white or sepia setting. Do you have a variety of value? Or do the objects and shapes become lost because they are all the same?
A good strategy is to use black and white paint samples from the paint store.
Complete a black and white sketch of the subject you will be painting.
Using the paint samples as a guide, find several colors for each of the values from black to white.
A good explanation of this can be found at this link: http://donnazagotta.com/blog/?p=4530
On thing this artist, Donna Zagotta, suggests is "Deliberate Practice". This is like the exercises I have been having you complete. I've also found it may be the best way to take my work to a higher level. She also discusses a movement called 100 washes. This is completing 100 different washes in watercolor but would also work with other media.