This is like collecting and publishing testimonials.
You could try this as part of the e-mail form on your website:
I would be grateful for your permission to quote from your comments in the forthcoming marketing activities that we have planned. If you are happy for us to do this, just sign/select the relevant option below. Otherwise, your comments will be treated in the strictest confidence.
Have a suitable check box/radio button that is nomally checked "accept" to maximise the opportunities for you or leave it unchecked if you prefer.
Add this as part of your privacy/usage statement, too.