If you do remodeling, especially larger jobs, be sure and offer HVAC duct cleaning at the end of the job. This shows your customer you really care about them and their home.
I know contractors that bury the cost of the cleaning in their estimates, others charge the customer for the service openly and still others pay the cost themselves. Whichever way you chose to go, be sure it is included. And make sure your client knows it's been done.
It's one of those little things that provides a great service, and helps put the home or building back to nice and tidy when you finish your work.