It can be difficult, but sometimes the best answer is no. Particularly because saying no to someone often can mean saying yes to your familia. If you have too much on your plate, don't commit to extra work or volunteering. Saying something along the lines of "I'd love to help with that if I didn't have so much going on at the moment" or "Sorry I can't make it, but thank you for thinking of me." Both are polite ways to decline anything you can't handle.