Do yourself a favour, while you are trying to collect customers email addresses, try and lose the manual process of asking people for the address and filling it in manually. Set it up on the net, a collection form and let your customers fill it in.
If you are using an email marketing software, like MailChimp which we link into it has built in such a form.
On your website, if you have one make sure that it has such an opt-in email form.
Now the idea is to use it everywhere to build up your email list.