No matter when or how you do it, just make sure at least every now and then you go a little out of your way to let them know truly how special and important they are.
This could be as simple as sending them random text messages stating how much you love them and how grateful you are for their friendship and for having them in your life! You can even just randomly bring it up when you hang out in person. Make sure they know how much you truly appreciate them and value them as not only your friends, but also as people. Make sure they know how important and special they are to you. Do not expect others to be there for you if you aren't there for them. Do not take their kindness and good deeds for granted.
