'Working a lead' is an unfortunate concept because the person at the other end often ends up feeling 'worked on.' Always remember that your leads are people who can tell you what's going on. Take an interest in their life—it won't take long until you relationship has a trajectory and a rhythm.

Marketing Strategy and Innovation Blog: Give Your Leads a Regular Check-Up, 2009-Sep-10, by Maria Pergolino:

How often do you ‘talk’ to your leads?
The frequency in which you engage with your leads is correlated with the chance of a sale. For example, calling a lead too much may frustrate the prospect and not calling them may cause them to forget about your brand.