One very successful method for increasing signups via your opt-in form is to offer a free gift that has value to the people you're targeting.
Typically this is in the form of a special report, an e-course, access to videos or audio, etc.
ASC member Bill Urell of Addiction Recovery Basics shares how he created a handbook of recovery resources as a free gift for opt-in subscribers, in this ASC forum post:
I Tripled My Email Signups with this Simple Strategy.
ASC member Myrtha Chang of Joy of Camping offers a free camping cookbook as a reward for subscribing via her opt-in form. To make her gift more eye-catching and desirable, she created an attractive e-cover image of her cookbook.