Plan your composition ahead of time. If you have a bad composition before you add paint you're going to have a bad composition after you have paint. You can change it after you've begun but it's a lot of effort and paint.
Prevent hours spent on a bad composition, make small versions of what you think you'd like your drawing or painting to look like. Doing quick sketches in small formats that are similar in proportion to your larger canvas will give you lots of information.
As a help for us, here are a couple of thumbnail worksheets for you to use in a rectangular format and in a square format. These are images, so you can save them to your hard drive by right clicking.
Here's a doodling assignmment: Draw three shapes in each square or rectangle that bleed off the edges of the square or rectagle. Then pick one to paint in an abstract way.