This may seems like a weird or obvious place to start, but think about it for a bit. If you receive an email about a service or product you're not interested in, haven't heard of, or haven't purchased in years, then you're likely to just delete it or flag it as spam. It's an annoyance and it makes people think less of you and/or your company. Make sure you are not the annoyance in someone's inbox.
The sure-fire way to be certain someone wants to receive email campaigns from you is if they ask! If you have a booth at an event, you might want to put out a sign up sheet for people who are interested, or put out contact request cards in your store for people to fill out and return to you. The easiest way to have people sign up is to use a signup form on your Website that adds their information directly to your subscribers in Email Marketing.
If someone has purchased a product from you in the last 18 months, then it might be a good idea to follow up with them. After all, they know who you are, they've clearly shown interest in your product or service, and a little reminder might make a repeat customer! However, if it's been longer than 18 months, then people might not remember where they purchased their product or service or wonder why they are suddenly receiving emails from you after all this time. It comes back down to not being the cause of that puzzling, annoying random email in someone's inbox.
No one wants their information sold or given away, even to companies who have products or services they may like based on their interests. Email addresses end up on these lists from signing up with companies who sell or share their information, or by having public email addresses on websites. Either way, it will be puzzling and irritating when they log in to their email account and find your campaign, as awesome as it is, sitting in their inbox without their consent. In fact, there's a good chance that a lot of what you consider spam in your inbox got there through someone selling or sharing your information. Once again, don't be the annoyance in someone's inbox.
Every campaign you send from Email Marketing has a Forward to a Friend link at the bottom so your readers can share all the great stuff you send with other people they know. Next to that, you can add a Subscribe link that takes them to your default signup form. That way, when people get your campaign from their friend, they can sign up on the spot!
Using these methods, you'll list will grow naturally and you'll have better results from your mailings. Also, you will have built yourself and/or your company a great reputation for treating people's information with respect.