Aside from sanding the substrate until it's perfectly smooth, viscosity and working time are the two biggest factors when working with varnish.
Some varnishes are ready to use right out of the can, boasting great self-leveling and working properties. Other varnishes may be a little thicker. For optimal flow and leveling qualities, these varnishes may require you to use their specified thinner or reducer. It is best to apply only the minimal amount of thinner needed to achieve the desired leveling qualities. Adding too much thinner/reducer can increase the chance for drips or sags, and even reduce the cured finish appearance.
Some varnishes are designed with a lot of drying agents, and may begin to cure very quickly, before the varnish really has a chance to self-level. Adding a very small amount of the varnish’s specified thinner/reducer can help resolve this issue.