Oh... Life goes on kitty. I am sorry. The thing about life is that people enter your lives, really make a diffrence, and move on. To make sure they stay with you forever is that you need to remember the fond memories you have with them, and remember what they taught you as a person.
Do not cry, or be sad that they are leaving. Rejoice--that that person may have a new life for him/her, and let him/her know that youll always be there. That is the best thing you can do.