Use a good matte primer and sand the surface until it's smooth. If you're using graphite or charcoal, go for the softest leads you can buy. Try to find 8B or softer. The fabric is more receptive to soft pencil or charcoal.
The best way to draw on a canvas is wit pencil, it is hard to erase so plan out what you want beforehand. You don't want to press hard either or it could poke through or stretch the canvas out.
