1) Let them go. This may seem obvious, but seriously, let them go. Don't try to talk them into a year at the local community college or insist they come home every other weekend. They are ready (well, not ready-ready, but as ready as we were at that age). They don't want your input and they will only resent your interference. Just let them go.
2) Cry. I once teared up over a coffee mug in a Marshall's (because I thought my daughter would like it, and wouldn't it fit great in a dorm room...Wah! She's leaving me). It will come and go and come again. Try not to sob around them, they don't need your emotional baggage--they need to go.
3) Shop. Apparently the way I approach any significant time away from my kids is to buy them stuff: food, laundry hamper, just the right coffee mug. For some reason my psyche is convinced that they will be fine if they only have the right bath towel. Whatever, it gives me something to do while I count down till move-out day. And it gets me out of the house where I can sob in peace.
4) Pray. Pray that you will survive their newly revived adolescent attitude. Something about that sweet smell of college freedom puts their attitude in over drive--everything you do is dumb, or boring, or parent-like compared to their impending campus life. You will survive this. Remind them who will be making those tuition payments, tell them to take the attitude down a notch, then go have a good cry in the department store (because they really need another set of bed sheets if they're going to be successful).
5) Delegate. Give them something to do. Again, they are restless and moody and need to be out of your face (But not too far--you don't have much time left with them! But definitely in the other room so you can't see them roll their eyes.) My daughter is currently painting our guest room--I'm paying her, mind you, because I really don't want to do it myself (and she's honestly does a better job than me). In the other room (I know where she is and that she's safe), not giving me attitude (because I'm paying her), thereby guaranteeing her safety. Win-win.
6) Silence. Once they move out, do not speak unless spoken to (which is also how survived my son's middle school years). Let them have time to spread those wings, fall a little, and try again before you start badgering them with questions. My son's first text message to me was two weeks after he'd moved out--he wanted to know if he really needed to separate his darks and lights when he did laundry. What I read into that? "I miss you, Mom, and I wish I'd paid more attention to you while I was at home." Yeah, he just had a laundry question, but whatever.
7) Cry (again). Once they move out, you don't have to worry about upsetting them anyway.
8) Look. Now that you had a good cleansing cry, look around at where else you can put your energy. Without the time spent on senior year activities (which I'm convinced are planned just so we parents will be happy to let them handle college on their own), maybe you could take up a hobby, rediscover your husband, have an adult beverage with dinner. And hey, I still got one more kid at home! Maybe she and I can go for a mani/pedi and then go buy matching outfits at Forever 21. Poor thing, she's going to be ready to move out by spring--too bad she's got 3 more years left of high school.
9) Enjoy. Yes, you miss them and you worry they're not eating right. But have you noticed there are no longer wet towels on the bathroom floor? And you can go all week on one jug of milk. Don't get me wrong, I LOVE my kids and miss them like crazy when they're gone, but I don't miss their bad habits. Even though I still have one more kid at home, I am now on the Easy Street of parenting--she can't complain that her other siblings don't have to do chores if they're not here. (And I've seen those pics you empty-nesters are posting of your last minute trips out of town, not a care in the world. I'm going to pretend you sigh heavily every time you come home to an empty house.)
10) Wait. They will come back--sometimes just for Christmas vacation, sometimes because they're broke, but they will be back. And they will be easier to talk to, and may occasionally clean up after themselves, and they might just even appreciate one or two of the millions of things you've done for them. My mother once claimed sending her kids away to college was all worth it because when we came home we put our dirty dishes in the dishwasher. Yes.
This parenting thing doesn't get any easier, the problems just change. As I watch my daughter pack her stuff I just have to keep reminding myself that this is what we've been working towards for the last 18 years. She is ready.