I've talked about this before. When you agree to sign up for something (anything), show up and do what you said you were going to do. This is being impeccable with your word (for those of you familiar with the Four Agreements) and also just something a good friend will do. Or, to put it more bluntly, something reasonable that you should do no matter who you agree to do it for (family, strangers, acquaintances, etc.)
I think we all agree to things at times when we're busy and we honestly think we want to do them but when the time comes for it to happen... we just skip it.
This behavior not only makes us look bad, it puts others in bad positions, too. You'll never know how, perhaps a year down the road, you will see or talk with someone that remembered you because you failed to show up when you said you were going to. Is that the way you want to start a friendship?