And never ever EVER get mad! Getting mad is a sign of weakness. Be firm, at the most. If you ever accidently get mad, or you decide to get firm - get positive and happy immediately after.
I'm speaking from immediate experience right now - I just had a girl flake, but turns out her phone only received the messages in the afternoon. She said she was really sorry. I tried to call but she was in the middle of baking and got her sister to hold the phone and relay what I said to her. I saw it all as being evasive and I said she sounds busy and ended the call. I then sent a stern message saying that she should have called me when she didn't hear from me cause we agreed to meet up, and that I wouldn't just cancel on her without a word, and that I was disappointed she thought so little enough of me to think that I would do that. She sent a longer message saying not to guilt trip her and that it was not a big deal that I didn't get back to her. It was hard but I realised (from past experience) that I'd reacted the wrong way, and I calmed down, and I tried really hard to think of the best, positive, non-wussy reply I could do in this situation. It turned out to be "Thanks for clearing that up. Enjoy your baking

I'm on holidays so I can do the [what we agreed to do] then. Cool?" then I even sent one right after that said "Tomorrow*" because I forgot to say that in the message. I mention this because people say it can look bad to send more than one message at once, but in this case I think it was justified and also I think the strength of the first message also made it okay.
There was a bit of a delay (like 5 or 10 mins) but she replied - "Yep tomorrow is good. Let's finalise a time and place now just incase technology interfers again. Heh. Asshole phone."
From then on we had an up-beat sms exchange about what we're going to do, and everything is cool. We already have a bit of a history and really made a connection recently which would have helped us get over that little hurdle a little more readily. But I believe that as soon as you catch yourself being a stick in the mud, it's never to late to switch to positive confident mode.