If you want a good varnished finish, make sure the environment is dust free. That means if you have been sanding, make sure your clothes and the brush you are about to use are dust free.
Use a vacuum to remove dust from the floor and wood; make sure the dust is not being blown into the air of the room that you are working.
When applying the varnish, use a light that will produce a glare on the working surface. This will allow you to see any runs or sagging in the clear finish.
A bristle brush will allow for slight ridges in the varnish. Using a foam brush will leave ridges on the edge of the foam brush. Slight thinning and brushing out the varnish will help solve this problem. Eliminate air bubbles in the finish by never shaking a can of varnish before application.