To encourage signups, keep your email registration forms short and sweet and ask only for what you need to get started.

Have you ever walked into a store ready to make a purchase, only to change your mind after nobody seemed interested in helping you? Or, conversely, have you ever received such exceptional service that it actually prompted you to purchase something that you hadn't planned on?

It's the same for the people on your email list. People sign up to receive your messages because they anticipate a relevant and enriching experience. When you deliver, they reward you with loyalty and high return on investment. When you don't, they walk away.

Here are five core strategies to encourage people not only to join your list but continue to respond over the long term.

Make it easy for people to opt in
Ask for email opt-ins on every page of your website. The more frequent and convenient you make your opt-in, the more people will use it. This is such an obvious point, yet I can't tell you how many companies bury their email opt-in forms deep within their sites. According to the "2005 Retail Email Marketing Study," of 175 Internet retailers queried by Silverpop, 23 percent didn't even offer an email opt-in on their homepage.

Also keep in mind that internet search engines drop users off in various locations throughout your website. So the ability to leverage paid search to drive visitors to email registration forms depends on the easy accessibility and visibility of an opt-in request.

Collect personal information over time
Your work is not over once someone clicks through to your email opt-in page. People still have to complete your form, and while you need to know enough to be able to send relevant messages, keep in mind that the more questions you ask on an opt-in form, the fewer people will complete it.

To encourage signups, keep your email registration forms short and sweet. Ask only for what you need to get started, and gather or infer additional information over time through relevant surveys and polls, email response metrics, purchases, et cetera.

The second thing to keep in mind is that registration forms actually can impact opt-in rates. For example, research conducted for MarketingSherpa's "Email Marketing Benchmark Guide 2007" shows that a double column registration form significantly underperforms a single column form requesting exactly the same information. So treat your opt-in forms as part of your marketing, and test to learn what works best for your target audience.

Manage your list
How do you allocate access to your list across your company so that everyone who wants to send email can, while ensuring that customers get messages at an appropriate frequency? Allocation can be accomplished in one of several ways. The simplest is to use a calendar to plot out timing of email sends up front. When one group is scheduled to send out a newsletter, for example, that time block is closed off to other groups to avoid overlap.

Another way is to allocate frequency by specifying how many messages a particular address can receive in a particular span of time. To help ensure that you don't bombard recipients with messages from across the enterprise, check whether your email service provider offers frequency management capabilities.

Maintain your list
The best, most anticipated marketing messages in the world do little good if they aren't getting through to the people who have asked for them. Assuming you have the basic technical configurations in place, the single most important thing you can do to protect deliverability is to practice good list hygiene. Hard bounces signify addresses that are permanently undeliverable, and no amount of re-sending will change that. But if you persist, you may succeed in getting your entire mailing blocked as spam. So remove hard bounces from your list immediately.

If you really want those people back, consider other means, such as using an email change-of-address or data-append service, or contacting them via direct mail or telephone. Thank them for their business, remind them of the value they receive from your emails, and instruct them on how they can continue to get your messages.

But you may want to consider just letting them go. A lot of people use secondary and tertiary email addresses because they don't want to be tracked outside the confines of your email relationship. They may have a very strong affiliation with your brand, and they may still want to get your messages, but they also want to control the longevity of their relationship with you. Let them seek you out on their own terms.

Relevance above all
Once people join your list, the key to keeping them on your list is relevance. Use each message to program recipients to open the next one. Make sure that each message you send is so interesting and beneficial that it will make the recipient eagerly anticipate the next one.

As you work to build your email list, remember that all lists will churn. Email addresses go bad as people move, change jobs, switch ISPs and forget addresses and passwords.

According to MarketingSherpa's email marketing benchmark guide, B2B email lists churn at a rate of just over 25 percent a year, and B2C lists churn at a rate of nearly 35 percent. The good news is that despite this turnover, lists are still growing at a healthy clip with B2B lists by 22 percent a year, and B2C lists by 37 percent a year.

Effective email marketing begins with an active and growing in-house email list. Campaigns sent to your house list will regularly out perform almost any list you can rent or buy. Carefully cultivate your house list and it will reward you over the long run.

Bill Nussey is the president and CEO of Silverpop, a provider of permission-based email marketing solutions, strategy and services. Read full bio.