I used to get just a TON of emails everyday asking questions about the service I was offering. I had an optin form on my site but they just were not signing up even though I offered a freebie.
Then I got an idea. Most of the questions I was receiving were the same questions and a request for a quote. So I did a bold move. I removed ALL contact info from the website. They no longer could email me with their questions. Instead I answered those questions and included a price list and made it into an ebook. Then I changed the optin form to say...download the FAQ and price list.
Now instead of receiving one or two opt ins per week I am consistently getting 30-40.
As I said, it may not work for every niche, but give it a try. Leave out a key piece of info and make them download it through your opt in.
Let me know how it does for you!