The beauty of a waxed canvas bag is that it looks and feels better the more you use it. Through use and adventures, the fabric picks up creases and marks that add character to your bag. Every mark tells a story, a story of places, people and experiences.
To make sure your bag will carry your stories for a long time, you need to take care of it. Here's how:

+ Don't overdo with the cleaning. A little dirt and marks won't hurt your bag. I promise. For major dirt like mud, let it dry and scrub it off with a soft bristle brush. If it needs more cleaning, spray on some cold water and use sponge. For really really tough dirt add mild soap (not detergent) to cold water, apply to the spot, rinse and let it air dry.
Do NOT machine wash, dry clean or use detergents, bleach or other harsh chemicals on your bag. It's bad for the environment and your bag.

+ Careful with the heat. NEVER ever machine dry, machine wash or use hot water to clean your bag. Extreme heat will break down wax coating and your bag will no longer be water resistant.
Of course, this doesn't mean you can't use your bag during the hot summer months. The wax adjusts to the ambient temperature, so you might notice that your bag will become a bit softer in hot weather. But no worries, it'll go back to normal at room temperature. But do be careful not to leave your bag in a hot car all summer long.

And remember, there's nothing wrong with your bag being a little dirty, scuffed and showing signs of wear. Those are signs of days well spent and bag well used.