If using your tent frequently, or for long periods of time, a cleaning and re-proofing treatment should be applied more frequently. Spot clean as necessary. The sooner a dirty or stained spot is dealt with the better. Dust and dried mud can be wiped away with a soft brush before a cleaning agent is applied.
Ideally after cleaning a waterproofing agent should be re-applied.
There are also various canvas cleaning specialists who provide this service. Do not clean your tent in a washing machine. This will damage the canvas fibres and may also damage your washing machine!