Making connections and networking is awesome, but keeping them? That's even better. It can be a little tricky to know what to do with established contacts if you are a little low on the activity scale and do not want to drop off the radar.
You need to think smart and be resourceful with your contacts, make them feel important and remind them in turn how important a contact you are for them. Anybody who knows what they are talking about will tell you that it is not always what you do in life, but who you know. There are two ways, which I think work nicely for keeping in touch with people; the first is a "catch-up" technique. If your connection is somebody geographically near to you, somebody whom you get on with, call or email asking how they are, and if they would fancy meeting you for a catch up. It's social networking. You're doing face-to-face conversation that will help reinstate the friendship you have or strengthen it depending on how concrete it was to start with. Even if you do not live near them, if you're working or visiting anywhere near, it could still be worth sending that invitation for a lunch date or trip to a good coffee shop. Even if your contact says that they're busy, they now know that you're thinking of them and it will remind them of your thoughtful personality, hopefully encouraging them to return the favour sooner rather than later.
The next method is sharing information. If you have done something that relates to them, something you're proud of, pass on the information! If it's worth being proud of then it will be more likely to impress. For example, if you and your contact met through a shared hobby, let them know of any recent (but relatively significant) achievements you have made in that area and it might just spark up a bit of conversation that will refresh your networking skills and help top up that rapport.
It's easy to see your contacts as just useful resources if you don't know them on a personal level. But they are more than a resource, they are a human with their own commitments, their own ambitions and their own flaws; treat them as such. Some contacts will stay with you, some you will pick up and some you will lose along your (hopefully!) long and rewarding career. The only thing you should be able to say at the end of it all is that you gave every single one your patience and your time when they needed you, and were always polite when you needed them.