Every single lead needs to be followed up with. The rule of thumb is, follow up until you hear a solid, "No Thanks, I'm not interested." This is tedious, we know. We also know how effective it is. Be personable, ask how they are.
If you've had a discovery call, make sure to follow up with anyone who you haven't heard back from yet! If you take detailed notes on your calls, look back on them and reference something they mentioned.
This is really ALL about relationships. People do business with people, not corporations. Remember this, and you will go far with your business.