Not everyone can be an administrative assistant. It takes a very specific set of skills to assist an executive or manager. The following soft skills make for a great assistant: time management, project management, strong communication, and active listening skills, as well as common sense.
Imagine if every entrepreneur had at least one key team member who could self-manage, continuously become more effective, communicate efficiently and excellently and still have the time and energy to lead a fulfilling life.
Finish my Christmas shopping -- with your own money. Wrap said Christmas presents beautifully. Find my sunglasses. Make a kick-ass gingerbread house that looks like the Witch's cottage from Hansel and Gretel so that I can look like all the other awesome moms on Pinterest.
If you think your boss is taking advantage and using you more as a personal assistant, then by all means, sound the alarm. But, start with the mentality that no task is too small or too big, and you'll be seen as a team player.