First, as soon as you find out he or she is moving, let it sink in. Think about it; they're going to be gone. Gone. From then on, spend every moment you can with them, do things that your friend likes to do even if you don't.
Do not blame his or her parents. Maybe that friend's dad got a better job, or maybe they just need to go.
Cry as often as you like, holding it in isn't going to help.
Make a goodbye card for your friend. Include that he or she was and still is your Best Friend Forever (BFF).
Create a special present for her/him to remember you. It can be a photo album of pictures of you two, a poem you wrote for him/her,or what ever you think of that would mean a lot to your BFF.
Write to them, call them, email them, IM them, all of that once your friend moves... you do not want to lose touch with that friend!
Make new friends, but keep the old.
Be sure to tell your friend that you will love them forever. If you two were really attached, then it may be best that you tell her/him that you'll call them every day (depending on your level of attachment) or every other day, every weekend, and so on.
You can even write a letter to make him/her special. Do not settle for printed copy. Mention the moments you shared etc. Make them realize the void in your life after them.
If you know how to write songs try to make one about how you feel about him/her moving.
On the day before he/she moves, order a pizza or something to share with them so they can spend their last hours with you.
They will also have other friends so let them be with them if they need to be as well.
Even though they are leaving, try to remember it's not like they have much of a choice. if they leave, you should have other friends to turn to. and i would like to say in my experience, when your friends move away, the best thing to do is STAY IN TOUCH!!!!!!!!!! when i moved from tennessee, my friends gave me a stuffed animal, a fake starfish, and mini vanity to color on the way there. they wrote on it, too. it says, "Come back and visit! we love you sister!!!" I have those thoughts when i get angry i simply get; happy. hope this comment was helpful!