1. Include it in the terms of service.
2. Depends on the product. If you sell anything that is considered to be hush, then don't. [personal massagers, married dating circles, etc.]
3. In general, people don't mind hearing from you. Though don't email them weekly, unless they subscribed to a weekly newsletter. Once every 3 weeks has proved to be a good start for me.
4. Make the emails personal, and answer them. The aim is to talk to people, not sell them something outright.
5. Resist the temptation to upsell. Why? People will then think that you are just emailing them to squeeze more money out of them. Having an email address like so: sales@startup.com, makes things easy. If anyone has an interest in buying, just tell them that someone from the sales department will contact them. Then send an email from that address and continue the dialogue.