Before each time a pellet grill is used its important to make sure the burn pot is clean of ash from the previous use. How easy/quick that is to do depends on the specific pellet grill/smoker.

On several older models you have to get the cooking grates out to get a shop vac in there to clean out the burn pot of ash. On other models, they have a quickly removable ash cup below the burn pot or a similar design where the burn pot can be cleaned of ash in seconds. 

The other important component to keep clean is the grease tray/drip pan. If you let that get a thick covering of grease, then the chance of a grease fire increases considerably, especially if the pellet grill has direct-flame access.

Other than that, keeping a pellet grill/smoker clean is the same as any other grill. You can use warm water, and detergent or a degreaser to keep it clean.