Part of being able to maintain a sugar relationship is directly attributed to your flexibility.
Understanding that your SD is a busy man is key. Working with his time constraints can make or break the arrangement. He is looking for someone who is able to deal with this.
Last minute cancellations are normal when in an arrangement. A person in position of power has last minute things pop up all the time. Rather than giving him grief about it let him know that you understand but expect him to make it up to you next time.
Sometimes it takes you moving around your social life to accomodate a last minute opening. He will appreciate this gesture and reward you accordingly.
If you have to cancel a date please give him notice. More often than not, the SD is a married man who has to be creative when it comes to allotting time to spend with his SB. Canceling can fuel suspition with his significant other and that is never a good thing. I usually cancel 48 hours before said date, then again, I rarely ever cancel.
The key is to facilitate his life, not complicate it. He is providing you with a generous allowance/gifts for your time. In return he expects that you are flexible and accomodating.