A. Try the ever tactful "I'm sorry, but my calendar is so packed right now that I can't fit in one more thing." The goal is to be polite yet firm and not to resort to outright lies, which only complicate things.
Maybe what you're really asking is how to say no without guilt. Remember that no one can make you feel guilty --it's a self-inflicted thing. And why should you feel guilty about setting a limit (once a week, twice a month?) on running other people's errands and caring for their kids? If you let them run you ragged, you won't have much left for your own kids.
To give yourself a dose of mom reality, make a list of everything you do in a day, from the time you open your eyes to the time you close them. Post it on the fridge as a reminder that you do, indeed, have a job. Be sure to give your husband a copy too.
Of course, if he works, it's okay to pick up his dry cleaning. But getting drill bits? That he can do himself.