Just Say NO!

So I tend to have a small problem getting the word "NO" out of my mouth. I have been struggling with this, particularly when it comes to school. People at school are ALWAYS asking me to help them do something, whether it is make a banner for their church (HELLO! Not school related!) or draw something for their bulletin board...I must have a sign on my forehead that says "I'll do anything you ask!" Today, I guess I needed a refresher course in saying no.
I've been giving a teacher a ride to school every morning, she lives in the same town I do and has no car at the moment. This is fine, and hopefully she will be getting her car fixed soon. ANOTHER teacher asked me today if I would give HIS sister a ride to and from school FOR THE REST OF THE YEAR! And get this, he wanted me to go and pick her up every morning (out of my way)and he wanted me to rearrange my schedule to take her home everyday at 4:00. Apparently she got a job at the high school and has no way to get there!! (Did I mention they are from India? So she doesn't drive.) I tried the nice way to tell him it was too much with me taking this other teacher to school, too. After all, I'm not a taxi service! He didn't accept that...and I ended up agreeing with him.
I walked away upset, not really wanting to become this taxi service, and feeling bad for wanting to say no. I love my morning drive BY MYSELF because for 50 minutes, I get to wake up and most days I sing and worship while I'm driving. I went to vent to my co-workers, who reassured me I should not feel bad about saying NO. So, I went back and told him I couldn't do it, and walked away. I said NO. (So I had to be given a refresher course) But still, I said no. It's a step in the right direction.
At the end of the day, one of my friends gave me an index card to carry around that says "NO". I think I'll keep it handy, and bring it out when I need it.