There's a great book on this subject: 'Smile or Die: How positive thinking fooled America and the world', by Barbara Ehrenreich. As the title suggests, she critiques positive thinking, but without implying that the only alternative is 'negative thinking'. She recommends (and I agree) that a better approach is simply being realistic - neither hopelessly pessimistic nor overly optimistic.
Sounds like a good read. I agree with that statement... I often feel like saying '**** it', but whats the point? Far better to take a realistic point of view of your circumstances, expect the worst but hope for the better.