get (one's) hopes up

1. To become excited at the prospect of something happening. Often used in the negative. Sadie has gotten her hopes up about going to the carnival tonight, so I think she'll be disappointed if we don't go. Don't get your hopes up about seeing Aunt Jen this weekend—I hear she's having car trouble. Don't get your hopes up—when the CEO says he'll revisit an idea next quarter, that means it's already dead.

2. To cause one to become excited about or expectant of something. Often used in the negative. Please don't get the kids' hopes up about Disney—we still don't know if we can afford it. I don't want to get your hopes up or anything, but there's a chance we might be hiring for a position in your field of expertise. I'm sorry, I might have gotten the kids' hopes up about going out for dinner.