The important thing with a newsletter is to make sure you provide content people want to read. Make sure the newsletter isn't just an ad for your services (although you should have information about what you do and what you offer clients in the newsletter). That being said, try to decide how often you can provide valuable content to your readers. It may be once a month, it may be 4 times a year. You also need to decide your budget for the project because mailing out a newsletter once a month could get pricey, depending on how many people are on your list. Once you do your newsletter, take it to the post office to have it weighed before you send it out. It would be really terrible to have them all returned to you for insufficient postage.
If you decide you want to do an email newsletter, make sure you understand everything about the Can-Spam Act of 2003. I wrote a blog about email newsletters and the Can-Spam Act that you can view
here. You can read all the regulations there and some tips, so I'm not going to retype it all here, but basically you have to make sure you have permission from every single person you send the email newsletter to, or else you can be fined and blacklisted. It's also important to make sure your newsletter is branded like your website and other correspondence so people recognize it as coming from you and don't hit the "this is spam" button because they don't immediately know who it's from. Make sure you send it out consistently, or else people can forget they signed up for your email newsletter and gave you permission and mark it as spam.
Hope this helps!