Every day for 5 minutes take time to visualize yourself at your goal weight/fitness level. Imagine how you will look, how you will feel, what you will be wearing or doing. The more detail the better.
When you find yourself worrying about staying on your diet or exercise program in the future, remind yourself that we are only in charge of the "now". If you take care of each hour the days and weeks and months take care of themselves.
I "psych myself out" when it comes to unhealthy food. If I see fatty snacks in the breakroom at work I think about how they are probably stale from being out or that a lot of people may have touched them. Or I will pick out one thing about the food that I might not like and magnify it.
I have found that doing a yoga DVD first thing in the morning gets me energized and feeling great for the day ahead. When I come home from work I actually WANT to take a walk instead of being exhausted.