Living frugally is an all-encompassing lifestyle that includes not just the money you spend, but the way you conduct yourself in regards to your other resources: time, energy, and belongings. In this case, the importance of saying no can't be stressed enough when it comes to spending those other resources; there are certainly times to say yes, but yes seems to be much easier for people to say, especially when dealing with people they care about. Knowing how to say no, though, is a crucial skill.
Over-commitment is stressful and expensive. Again, set aside monetary means for a moment. Even if you don't have to spend a cent to help someone out, if you commit a significant amount of time to him or her, you may be giving up valuable time you could be spending on repairing your own home instead of paying a professional to do so, or hours you could be dedicating to a side business to make some extra money. Even if the time spent on someone else's projects is just stressing you out, that could lower you productivity at work.
If you don't say no, your priorities could be rearranged without you realizing it. You are probably pretty clear on your priorities in life: your family may come first, with your job coming next, or whatever order you've chosen. When you aren't able to say no to others, you start putting them before your real priorities, and then your life gets skewed out of perspective and order. Say no, unless you know for sure you won't be sacrificing something important.
You can burn bridges just as easily by failing as by saying no. A lot of people cite "I don't want to burn any bridges" as their reason for saying yes to projects at work, co-workers' requests, family members' pleas, and more. However, if you promise something and then simply can't follow through on it, that bridge can be just as easily burned. If you aren't sure you can complete something, just don't say yes!
It's possible to say no without being rude. Here are some pointers on ways you can say no without sounding like a jerk:
"I'm overcommitted right now, but hopefully I can help out with future projects."
"I may not be the best person for this job -- have you asked [suggestion]?"
- "I'd love to, but... [reasons]"
Say no to overpriced garage sale listings! Use GarageSaleCow.com instead.