Exactly this ButterflyEffect - hope for the best, but prepare for the worst. I also try not to believe that what i want to happen is going to happen, especially if the odds aren't in my favor. For instance recently I applied to grad school, and I knew it was much more realistic to believe I wouldn't get in. I would so much rather be ecstatically surprised because I got something I wanted, than blase because i got something I felt I "deserved."

So, uh...expect for things not to work out the way you want. Sounds a little negative but honestly, it is pretty important to practice realistic expectations, or else you end up that crazy Bridezilla where everything is planned planned planned and perfect perfect perfect and the day will never live up to your expectations, ever, anyway, at all, because they're just so completely sky-high. (Bridezilla was the first good example i can think of, but basically, anyone can do this about any event. Birthdays too probably. These people end up usually awfully disappointed, a result which is inevitable because they started with expectations that could never be met.)

Don't be like that. Be like the opposite of that, about whatever you're excited about.