The bottom line is that running errands for your boss takes valuable time out of your workday. This causes you to get behind on your own work and may cause you to miss out on valuable professional opportunities at the office. If you're a person who just can't say no, don't fret. There are professional ways to solve the problem with your boss. It only requires preparation and good timing.
Timing Is Everything.
Next time your boss tells you to run a personal errand, use the opportunity to raise the subject. I do as well. I need to finish that XYZ report you want by Friday. If this backfires and your boss tells you to hand the project off to a colleague, stand firm. Tell him that in order for the report to be finished properly, you must do it.