I have found that if the affirmation isn't believable, then it won't work for me.
I definitely agree with this. In November 2007, I joined
43 Things and listed all of my goals.
At first I had over 25. I'd go back regularly to read the goals and say affirmations. Now a year later I'm down to 14 goals because most of them just didn't FEEL good when I went to affirm them. If the goal/affirmation doesn't feel good, I believe that it's my inner voice telling me that either the goal is not right for me, or that it's just not the time to be thinking about it.