A few weeks ago, I was coaching a sales team when one of the senior reps asked, "How do I get someone to respond to my calls or emails? I've sent my prospect information, but now I can't get in touch with him." The short answer is you don't.
You can't force someone to reply to an email or return a call. But if you arrange things properly, you'll never have to call or email to say "I'm just following up." The key is to establish a day and time for a subsequent conversation before you send your prospect information.
Here's the approach I use to reconnect with my prospects after an initial call or subsequent discovery meeting: Before I end the call or meeting, I say will something such as "John, I will get that proposal to you by Wednesday. When is a good day and time to review it with you?"