Setting Limits

How do you set limits on activities you and your family are involved in? I've really struggled with this and tend to overload my plate. I find that my home is in better shape when I'm not on the road. We don't go to every homeschool event or we would never get any homeschooling done.
I've recently cancelled a breastfeeding support group meeting. At first I felt guit, but the strain on my family was too much. Plus the fact that I've had to cancel numerous times due to the illlness of one of my children. There are just some things you can't plan like 2 am stomach virus on the day of a meeting. I just cleaned out the shelves we had at the meeting location and I just felt relief.
I want to have time and a desire to do the things God wants me to do.