Wow. 6-8 hours from start to finish is quite amazing.

For me dealing with client is always the most difficult part. Im a techie guy and I feel much better dealing with hardware :) What I've learned is to be as precise and direct as possible. Lots of clients have no idea how you work, and won't know how much effort some changes take. It's good to give examples of what you consider a major chage that would require additional charge, and what is a small change. Writing it in emails is much better than just saying it, people ten do forget, misinterpret etc and having it in writing always helps. If I meet the client in person I always follow up with an email to sum up what we discussed and make sure that we both understand each other.

Hope it helps