Ok, before you do something WAY stupid, let me explain how exactly you have to do this for it to work: You have to feel deeply desperately in love with someone, or else this is just something that doesn't work right, and you have to mean every word of it. This person finds someone or something that enchants them and makes them want to leave you so that they may have it (going to a foreign school; dating another person, etc).
You tell this person that you love them, and if it makes them happy to do that, then they should do it. You're happy if they are happy. And if later down the road they should come back to you, you will welcome them back with open arms. This means that they love you just as deeply as they love you! Hint: This only works when you are REALLY in love and REALLY mean every single word. And if the same person takes advantage of this more than once, it is not meant to be. They are just using you.