I do believe in positive thinking, and Im sure it could help, depending how much time you give to it!! I know for me I told myself I'd be early and even changed my date that I told people, and I was 8 days early!!
I think positive affirmations are important as well.
during my labour I actually talked to myself and said things like "you can do it", "this will not last long" "the pain is only short term" etc, and I was really aware of calming myself down. Totally unlike my first birth! I think it really helped talking myself through it (all silently) and being positive about it! xo