I find it helps to think about composition as visual design. Forget about the actual subject matter, instead focus on how you're going to divide up the space and arrange the biggest shapes to make it interesting.
Somehow my landscapes always start with the horizon across the middle. This is to be avoided! Unless you’re aiming for calm symmetry, emphasise the foreground or the sky, making the horizon high or low. Keep detail to the lower third.
Popular compositional devices include:
the diagonal layout, but don’t let it run out at the top of the painting; block it with a strong shape or vertical line to keep you in the picture
the “L” layout and inverted “L” layout
the “S” zigzag layout that leads you into the painting, across and out again
the “O” layout with a diagonal line to lead you in
cropping to create a sense of motion by putting you right in the midst of the action
Consider the atmosphere you wish to create:
symmetry fosters a sense of calm but can look boring
an asymmetric painting will feel more lively and dynamic
one that is unbalanced (e.g., that’s heavier on one side) will induce a feeling of unease
horizontal lines create calm divisions of space
verticals are more punchy
Place your focal point(s)
You want your eye to rest on the most important figures, objects, buildings or trees, so place them two-fifths of the way across the painting.
Or use the "rule of thirds” - divide the board area up into three rows and three columns, and place your focal points where the vertical and horizontal lines meet.
Create leading lines (a fence or avenue of trees or the curve of a bay) to direct the viewer around the painting.
Visual design also includes colour planning*, value planning and so much more, but I will save all that for another blog.
Keeping things simple is the key to good visual design. If your composition isn’t working, try to mass shapes together, removing detail.
Reduce the light in one area and increase it in another for greater contrast. You will be surprised at the difference it makes.
For visual impact, make sure your darks are properly dark and your lights are truly bright.
*See my blog on how to choose a colour palette