You need to realize that friendship works the same way as every human relationship. It needs energy to keep it up. Especially after the honeymoon period (which is usually longer in a friendship than in a romantic relationship). You need to give in the relationship and you need to gain from it everytime you are together. Don't think about big things, but take care about each other. If you share your deepest secrets with each others, if you rely on each other, time will make you "better friends". The best thing in real friendships is the fact that you became each other's "other half" without even realizing it.

To cut the long story short: There is no special recipe for it, but if you will be there for each other, life will get you closer and closer. ( The older you are friends the better in my opinion.) :)