A few points for shops:
1) If you think you cannot do a job, be honest with the customer and direct them to a shop that knows their shit for that particular job. Don't try to do things you don't know how to and screw up what could have been a great relationship with your customer.
2) If you are going to give time-lines to customers, stick to them. If you cannot meet it, reschedule only once. Don't make it a daily/weekly thing to postpone because you decided to accept business you could not fulfill on time. If you don't know how much work is involved, see point (1) above.
3) Do your research before you give sub-optimal options to your customer. You are running a business and your biggest advertisement is word-of-mouth and forums. If you are unsure of options to present, see point (1).
4) When a customer accepts all your quotes without question, it means the customer is placing a lot of trust in your experience and work quality/ethic. Do not betray this trust. These are also the customers that will help you succeed as a business. Remember, you need them just as much as they need you.
5) Provide discounted rates to customers who are willing to pay by cash.
a personal wishlist item, ha ha. Also treat repeat customers with utmost respect and stick by your promises/estimates. If a customer is coming back, its not time to rest back on your laurels, its time to do even more than the last time to make them happy.
I'm sure I can think of some more, but most if not all of these suggestions stem from my personal experience with the thousands and thousands of dollars I've spent.