It's so hard! This is my third summer away from Rain - we were together last summer (after 9 months apart) but apart the two summers before that. I still miss her a ton, but we're more used to things now so we kind of have a plan. Does your daughter have email? Skype? We pretty much email every day or two, just checking in, as long as we both have access. We skype when we can, too, and I send at least one postcard. I'm assuming your daughter is in the same country, too, so you could even call or text... if the camp allows it, I guess. We did run into problems with a CTY camp as far as parent contact, but I'm not sure how typical that is. I would try to be sure that she does have a way to contact you if she needs to.
I find it helps me to talk about my daughter to people... it makes me feel closer to her. The first few days are always really hard but then it gets easier, IME.
This year Rain is actually a camp counselor, so she's dealing with homesick campers herself...