Care should be taken with your stretched canvases both before and especially after they have been painted.
Your canvases should be stored vertically whenever possible. If they are stored angled against a wall they may warp over time. If they are stored horizontally they may sag and become unusable. Sagging canvases can be re-tensioned but this can be avoided is they are stored correctly.
Keep your canvases in a temperature controlled environment such as your home or studio. Do not store them in out buildings such as garages or sheds where they may be susceptible to moisture. This can warp and damage them prior to use.
If possible store your canvases in darkened areas and not in direct sunlight or near heat sources.
Multiple canvases should be stored back to back. Another option is face to face. This minimises the chance of canvases being marked or stained. If possible use bubblewrap on completed paintings.
Framing your finished artwork can protect a painting and reduce the chances of it warping in the future.
Always make sure you choose the ideal canvas for your painting. Lighter canvases can be affected by just a wet or thick wash of paint. Remember you are dealing with natural materials that will react to temperature changes or moisture.