The best way to preserve a work is always... to frame it! Other methods will, however, allow you to protect the work, both from accidents and from the passage of time.
Certain varnishes are indispensable, such as those used by restorers to separate their "repainting" from the original work. Most varnishes are optional and have their proponents and their detractors. If they noticeably change the brilliance of pastels, for example, they also improve the adhesion of their pigments. They keep powdery media (dry pastels, charcoal, graphite pencil for example) in place during regular wiping, eliminating soiling or dust. In short, they unify the pictorial layer of the work and protect it from oxidation as well as from possible impact.
Learn to stretch your canvas!
In every case, and even if paper is less sensitive to variations in climate than canvas, protect your treasures from wide variations of temperature. Like a good wine, artwork will age gracefully in a stable atmosphere, neither polar nor tropical!