PychtodayAffirmations have been around for a long time. Most people use them to shift negative beliefs and to feel more confident. 
I found a great article posted on the Psychology Today website that shares recent research on the effectiveness of affirmations. The article shares both the positive findings and the controversial ones.
Carnegie Mellon's research showed that affirmations reduced stress and increased problem solving performance. Another study showed that affirmations help us "deal with threats to self-integrity". The  researchers who question the efficacy of affirmations say that there is evidence that repeating overly positive self statements can actually make people with low self esteem feel worse. 
It's an article well worth a read.