Once the underpainting is dry, fight the urge to dive right in with details! This is a beginner's most common mistake and leads to stilted, stiff paintings. At this stage, you should be modeling the different shapes with color, light, and shadow. Adjust as you go along, keeping the whole piece in mind and in view as you work. When you have the canvas covered in paint, let it dry again.
Next session, when the paint is fairly dry (enough to have some tooth), start work from back to front and refine your shapes, emphasizing the direction of the light, and the depth of the cast shadows. Remember to add color to those shadows by tinting them with a complementary of the object's color. Your lightest and most saturated colors should be in the foreground. However, you should not have much sharp detail, if any at all, in the direct foreground. This keeps the viewers' eyes from traveling around the painting.
Okay! Whew! Now you can add all those details you've been dying to add! Just don't overdo it. Less is more. Keep some soft edges; blend like crazy in the sky and in the water. Save your pure whites for the very end, and use them sparingly. The viewer should see the light on the subjects as if he were standing there with his hand shading his eyes from the sun.