First, you need to build a list. Not any list, you want a precise, categorized list. There are several ways to build your list. The easiest is to start with your current customer base. Once you have this starting point, you'll want to segment your contacts by date. Putting customers into buckets gives you an idea of the "purchase commitment time" they have. For example, some customers may remember you and want to buy from you 3 months after their initial contact. Others will need more frequent reminders to keep them on-board. Once you have your current customer base setup, you'll want to send a notification asking them to "opt-in" to your newsletter. This is a soft way to get your foot back in the door, and ensure you're getting explicit permission to send them correspondence.
Getting new customers emails is the second phase of building a list. There are several ways to do so, and the most reliable is creating a "join my email" button on your website. This allows anyone that happened to find your website, as well as those currently familiar with your business to sign up for email updates. If you have a service provider that gives you options, then take advantage of trying to collect as much information as necessary to later categorize your new email customers. Other options for collecting new emails is to create an actual sign up form at your front desk. Ask people to join. Avoid doing "raffles" due to the lack of commitment by those who typically sign up. You want solid, interested customers receiving your correspondence.
Now that you have a list, when should you mail? Studies show you should mail frequent enough to keep customers informed, but not too much to overwhelm them. It's a good idea to pepper in business related ideas, along with your normal articles to give your readers "value" outside of just a sales pitch. Most experts say the best time to mail is on Tuesday morning. This gives your customer a chance to read their emails from the weekend, get their week started, and take time to peruse non-immediate email content. Other studies have shown that Sundays are also a good time to send emails, as people who keep up on their email, will often check in on Sunday to see what the week has in store for them.
You've got the list, you know when to email, so how often should you send emails? This is very particular to the industry you are related to. For example, a restaurant doesn't worry about too much "special" offers, because everyone needs to eat. However, a tax company will probably not get much response outside of the typical "tax time". What's most important, is your reliability and consistency on sending mail. You'll want to setup a timed schedule, so that your customers become familiar and look forward to getting their "monthly email" on the first of the month each month.
There you have it, the email marketing basic steps. Get a lists, make it specific, send important, related content on a consistent and frequent basis. Good luck!