One tip from Bob Ross' "The Joy of Painting" is to use a thinned-down white paint to give the blank canvas a thin base coat. While the base coat is still wet, begin to paint. Doing this helps to create more variation amongst the colors applied to the canvas.
Another tip from Bob Ross' "The Joy of Painting" is to drag the brush through the desired paint color on the palette, tap it a few times on a clean part of the palette and then repeat this process a few times. This ensures that the paint is evenly distributed through the bristles of the brush, preventing unwanted darker streaks from appearing on the canvas.