Saying No

For some reason, I have the hardest time saying the word "no".  Because of this, I somehow find myself overcommitting and loosing sleep. However, this year, I've slowly started saying "no" more often.  For the first time in college, I've been getting about 7-8 hours of sleep a night during the week!  Of course I am sad sometimes because I do miss out on hanging out with friends late at night, but I'm able to manage my time better because I wake up earlier.  For school, I'm waking up between 7 and 7:20 AM.  On the weekends, I usually sleep until 9 or 9:30 AM.  Usually I would say that waking up at 9 is just horrible, but because I have been getting up so early during the week, 9 AM is officially sleeping in.  However, when there is a choir concert in the morning, I do sometimes have to wake up earlier than I normally would on a school day.

Does anyone have any other strategies to control yourself from overcommitting?? I'd love to hear them!