I really like this question!
After a few too many times saying "yes" to something I was too busy for, too tired for, or not properly equipped for, I realized that I would rather come back and complete a job than do it half assed and make myself look bad. I will tell the customer straight that I am too tired to do this today, I will have to come back tomorrow to finish as I am not going to do it half way or anything less than perfect. This situation is obviously when I am already onsite and the work is more than expected or the scope changes.
People really respect it and never give any push back. I think the honesty is what they appreciate and knowing that I am not will to give them anything less than perfect.
I completely agree about honesty. I said to my wife tonight, "Being honest about something to someone when you don't really have to lets them know that you're going to be honest on another occasion when you could have easily lied about it."
