1. Everyone you meet is important. Our industry is a small one. People change jobs all the time, and you never know when you'll be working with someone again or for the first time. Another truth is that you never know where these people will go, especially when you're still in school. That guy you hated to work with on group projects in school may very well be your rep or your boss. Don't burn any bridges and consider every connection extremely valuable.
2. Automate your address book. If you're going to build any sort of meaningful relationship, it's going to require some work to grow and maintain. When I was looking for a job and needed my network the most, I realized too late that I hadn't been in touch with many people in a very long time. Here's a tip: Create a system where you contact at least five people every month that you haven't talked to in a long time. If you automate this and make it part of your routine, your contacts will never fade away.
3. Adding value is essential. Everyone knows people who reach out to you only when they need something. I've definitely been guilty of this offense, and it's very obvious when people are doing it to me too. Every time you contact someone, make sure you're adding value. It can be as simple as sharing an interesting article or introducing them to someone new. Remember that your network is only as strong as the value you add to it.
4. You're building relationships. Finally, remember that you're building relationships first and a network second. It's easy to think of your network as a tool to be used at your disposal. This is the absolute worst way to think about it. Relationships are the foundation of a long-lasting and mutually beneficial network. If you forget that your network is full of people just like you, then all they'll ever be is someone you've met -- not someone you know.
Unfortunately, I definitely don't always do these things. But I should, and you should too. My professional relationships would be much better if I did.