To prevent dust from getting on the lens, especially if your shooting outside, you can flip the eyebrow of the mattbox down to protect the front of your lens between shots. Don't forget to always carry lens wipes as well!
2) Setting up for a new shot
Think about position, height and always remember to BALANCE. Check framing and focal distance, focus, than double check exposure.
Make a check list in your head, or on paper, to make sure you have thought about all aspects of the shot.
When setting up lighting, don't always go for what looks good. Think about the mood of the story your telling, and what light will achieve that. Whether it is hard lighting to add intensity or soft lighting to create a calmer atmosphere. Always have a reason. Think about the scene itself, the characters and the dialogue.