We include an opt-out of communications option on our membership form. In addition to that, this year we began putting an email signup sheet on the PTA table for back to school night and open houses. It seemed to work very well. And we now have about 470 people who receive our email messages.
We don't have a PTO website and our school website is sadly underutilized. I created a form to send home to get parents (non PTO members) to sign up for our PTO eNews. I just uploaded it in the file exchange. It's titled "eNews Parent sign up form". Here's the link -
If your parent association has a website, you could make the process very easy by adding a feature to capture contact information so parents can register their email on the site in order to receive school news.
Rather than creating a form, distributing the form, receiving the completed forms, and entering the data on the form, your steps are reduced to adding the capture feature on your site and promoting the option to parents so they'll register their emails there.