I have been applying to the U.S. Navy and was again reminded the value of having strong references available.  The Navy requires 5 references, plus 1 from work and 1 from school.  This can be an absolute nightmare if you do not have people that you can easily use.

I graduated undergrad 5 years ago, back when we had a strong economy and I never thought I would need education references anymore.  I kept in touch with a few professors for a couple years, but as the time went passed, so did my keeping up with the professors.

Fortunately, I have been keeping up with my grad school professors and directors.  Not sure what I would have done if I did not have been in contact with them.

Moral of the story: Keep in touch with professors and other people you meet.  You never know when you will need your network.

What are good ways to do this.

• Get contact information early: try to make a connection while you are still in class or working with them.  Worst case, when people are leaving, it tends to get sentimental and is a good time to exchange email and other contact info.
• Follow up promptly: once you have someone's contact info, shoot them a quick email, nothing difficult here.  Just one or two lines and mention something that you were connected over.  This could be a joke, a class, a book, almost anything, just something that you remembered or noticed while interacting with them.
• Reconnect seasonally: for some reason, quarterly is a great time to connect with people.  It can be as simple as just saying hello and a quick update of what is going on.  Even better, with social media, maybe just a quick tweet @ the person or a simple Facebook message will suffice.  At least do this once a year, and if possible a birthday is a great time to connect.
• Keep a master list: It is great to keep all your information organized, this will seem completely pointless until you need something.  Have a view of all people you have connected with, like a mini CRM for yourself.  Facebook, could do this for you, through lists.  Or even in Gmail with Labels, but I have found a quick little spreadsheet or database does wonders.  Spend some time on this, it will be well worth it.
• Now it was worth it: the relief of having done this work pays off when you need a reference or looking for a job.  Over time, this list can grow into something very valuable to you.  It is a great way to network and you never know where your next opportunity will come from.