Did you know that you can keep notes on connections right on LinkedIn?
Find the connection, click the Relationship tab under their headline and choose any or all: Notes, Reminders, How You Met or Tag.
Add the notes that you need that will help you keep track of that person. LinkedIn also keeps tabs on any articles, etc. that you share with that individual.
Especially useful with those, like my colleague Maria, that I call on often in one way or another.