I would fill in "being busy".
Others might put "having kids", "moving away", "disagreeing", etc.
No matter what your reason is, my tip is the same:
You keep your friends by investing resources to keep them.
When we are young, we spend a lot of time building our friendships but as we get older, we tend to drift away from our friends for a multitude of reasons. If you want to rekindle or keep a good friendship, here are a few practical tips that has helped me:
Call Them
Initially, I felt hesitant to call up a friend I haven't spoken to for a while but I thought about how I would feel if a good friend called me up out of the blue to see how I was doing. I would feel good because that person had to be thinking about me before making that call. When you take time to call someone, you're telling your friend that you've been thinking about them and that you care about them.
I called a good number of my friends and for the most part it has been a great experience. At first, some of my friends were confused because they thought I wanted something but once I explained and called them a few times, the initial awkwardness faded away. I am glad that I did it.
Meeting in person is also great but not always practical. Email is convenient for finding a good time to talk but a bit inconvenient when you start going back and forth. I find the phone to be a good balance of convenience and personal connection.
Make Yourself Available
Good friends are there for each other in times of need but as we grow older and start our own families, we tend not to be available any more for our friends. We have our own commitments and dependents and it is easy to see a friend's need as a nuisance. This brings me back to my initial point: your friendship is as good as the investment you put into it. If you don't want to spare the time, don't expect to keep an awesome friendship.
The way I make myself available is to be in touch (see tip #1 - Call Them). This will help me become aware if my friend is going through difficult times. It is rare for a good friend who you've been out of touch with to call you out of the blue for help. Sometimes it is hard for a good friend who you are in touch with to ask for help so I need to be sensitive to my friend's situation and offer to help if necessary. This also means that when you are calling your friend, you are asking about all aspects of their life and not just talking about Jeremy Lin and the NY Knicks.
Get Together
Good friendships are not only solidified during tough times but also in good times. One way I keep my friendships strong is to get together. I find the group dynamic an excellent way to create new enjoyable memories with my friends. I have recently created some great memories at a few weddings and house parties.
The easiest way to get together is to host one. It may be more work but you can control the guest list and you don't have to worry about getting home too late. If you don't like to host, restaurants and lounges are nice places to meet as well. There is really no excuse not to organize a get together with your friends.
If you've read this far, you are definitely interested in strengthening your friendships. I recommend that you take out a pen right now and write down the names of 3 people you plan to call within the week. You're going to be glad you did.
I must admit, calling is a bit tough these days but what I do enjoy are group gatherings. Everyone becomes a bit disconnected after college because everyone has their own life in different locations which makes it even more difficult to keep in touch. What I find helpful is facebook. This social media tool has kept me in touch with most of my friends/family and it has brought everyone a bit closer. It’s easy to drop a line to see how things are going. I would also send an email or if I know I’m in the area, I’ll try and see if they have time for a coffee. Making new friends can be tough but maintaining old ones may be tougher though it is really rewarding once the connection is rekindled.