Breaking up...with a friend

Breaking up is so hard to do. But, parting with a friend is even harder. 

I remember when I was in my 20's and I had a close friend who I use to party with every single weekend without fail, we literally did this for years. It was great. But, eventually we just stopped catching up and spending our weekends together. Nothing was said of it, it was just a mutual parting of ways. Without any drama. 

But what happens if you find yourself drifting from someone much closer, someone you've had a deeper connection with for years if not decades? What can you do then? Friendships are often hard to define and sometimes even harder to part from because there is actually no clear lines as to what is acceptable and what isn't. 

There is the route of being incredibly honest with them and say your bit and that you are moving in directions or you can try phase the individual out, stop returning calls but that seems a little to abrupt to someone you have shared so much with. 

So what is the right way about breaking up with a friend, a great friend but who you no longer see eye to eye with? And there is no big dramatic event or wrong doing to justify the parting of ways. 

Is there anyway to do it with out great offense or is it a big black hole that no matter what you do, you may likely just end up offending. 

It's a very tough question to answer as with other relationships, intimate relationships there are generally agreed rules as to whats okay and what's not. So, if one breaks those rules it is reasonable to think that the other may just part ways. This is universally understood. 

But is this what friendships need, clear expectations, clear understandings or are friendships just a slow moving oceans of comings and goings, what is right in one part of life is okay and if another time things change, you just drift away into the great abyss. 

Have you experienced a parting of friendships?