In order to close business you actually need to interact with potential customers. And leads are one way to do that. I break leads down into 3 groups. The first group are lists. If it's a big list and you send them a postcard, good luck on getting a 2% response rate. If you send them an email, that's spam.
The second group is a response list where in order to get some sort of information, they have to register. You should get a higher response rate from this group, but I recommend a passive approach. Just because they attended a trade show, downloaded a white paper or gave you a business card does not give you the right to bombard them with email and phone calls. The best approach here is to followup with a short email thanking them and give them the opportunity to opt in. Then stop.
The last and my favorite group is the ones who contact you and want to buy your product. And you better get back to them within a couple of hours or a day at most.
Our credit union has a car buying service. Since a family member is in the market for a new car, we tried it out. We sent requests to 3 dealers each for Toyota, Mazda and Hyundai. Out of the nine, only two ever replied. The other seven just didn't care to sell us a car. Now I don't know about you, but last I heard auto sales were down and everyone is hurting. So when a well qualified customer wants to buy a car from you I don't think ignoring them is such a good idea.