I have learned, slowly and painfully, that I need to wait 24 hours before saying yes to any optional assignment. I'm too optimistic about my available time and wired to be a people pleaser. When I'm asked to help and I recognize a gap I can fill, I tend to volunteer, even if I'm not the right person for the job. Days or weeks later, I realize it was the wrong decision then the Ugh feelings start up in earnest.
It's important for me to realistically assess whether I have the knowledge, desire, and time to do the task. I'm learning to say no more often than I would like, and to be less afraid of the consequences of saying no. Promising to do something I can't or won't do is bad for everyone.
Better to think on the decision and politely say no up front if I don't have the time or interest or enthusiasm required to get the job done.