Are you one of those who has a drawer full of business cards from people you have met only once? An immediate email follow-up makes sense, even on the basis of “it was good to meet you”.
But even if you do this shortly after the first meeting, it is easy to lose touch, even with people that you have dealt with on a regular basis. LinkedIn is a good way of reconnecting, firstly through inviting inactive contacts to link. For those who are already contacts, work your way down your list, contact them and suggest a chat by phone, or a coffee as appropriate. The trick obviously is to keep contact sufficiently regularly to stop rigor mortis setting in.