If you're like most marketers, your house email list is growing at a healthy 38.4% per year, but there are still at least a few stragglers in your target market outside your email net.

Why not approach them by popping an ad in an email newsletter they already signed up for and presumably enjoy?

The problem is, email ads are awfully hard to make work. Most marketers make a critical mistake with the creative (detailed below) that reduces response. Beyond that, most marketers don't understand how the art of media buying applies to newsletters.

If you rely on an agency to do the buying for you, chances are they're not experts either. Even interactive-focused agencies are less than interested in perfecting their newsletter media buys. It's too much trouble for the typically small commissions.

So, you'll need to give direction to whoever does your newsletter media buying. Here are MarketingSherpa's top five tips:

Tip #1. Determine true reader engagement

Unlike most media buys, newsletter advertisers don't get any guaranteed impressions -- even half-hearted ones -- from the total list. So abandon your CPM (cost per thousand) thinking and don't assume big lists generate more impressions than small ones.

The only eyeballs you're buying access to are the ones that bother to open the newsletter. Depending on your industry niche and the brand power of the newsletter, that can range from 25%-40% as measured, with a few percents tacked on for non-measured opens.

Plus, many recipients only open issues for handful of seconds to scan the newsletter or just the top headline. Your ad may get a microsecond's attention if any at all.

So, in addition to demographics, media buyers must judge reader engagement. How thrilled and excited are these consumers about this particular newsletter? The higher the engagement level, the more likely they'll open and review the contents more carefully. Plus, if you're the lesser-known brand, the more likely your ad will gain from the implied endorsement.

-- Permission status and engagement

Did the names on this list specifically request to receive this *particular* newsletter, or did they say "yes" on a generic opt-in for "news" from the media company?

In particular, B-to-B trade publishers are infamous for launching new newsletters off the backs of previously-gathered lists. They just start sending readers an entirely new newsletter out of the blue, with an "opt-out if you don't like it" clause. It's not true permission and does nothing to ensure reader engagement.

-- Time of day for send and engagement

Many traditional media companies still have no understanding of how time of day will affect newsletter ad response rates. Their send time is dictated by when the editorial team can get the issue out, not when the recipients are most likely to eagerly open it.

And, especially with daily and weekly publications, a late-afternoon newsletter may not get the reads it deserves.

-- Interactivity beyond clicks and engagement

While you should ask about click rates for stories and ads, you should also ask about other brand involvement indicators. How many letters does the editor get after a typical issue? What sort of response rates do they get to reader surveys, contests or polls?

Also, judge for yourself, does the newsletter have a strong human voice or opinion that might boost reader's emotional involvement?

Tip #2. Frequency clumping

Don't make the beginner's mistake of running your ad in a single issue as a "test" and then judging your results from that. Instead consider two factors:

-- Opens are not the same people issue after issue. Not many recipients are inflexible in their reading habits. Some issues they'll open and some they'll never get to. A newsletter with a consistent 30% open rate probably is opened on a routine basis by at least 50% of the readership. If you advertise only once, you don't reach them all.

-- Multiple impressions get higher response rates in every media ever studied from TV ads to banners. The "perfect" number of impressions per prospect is roughly seven before response rates start dropping. You'll want to test to discover the multiple sweetspot for your own offers and brand.

If you're running the same creative across multiple media in a niche market where you're very sure prospects will see the ad in several places, you can get away with fewer ads per newsletter title. However, don't assume the ad that got the click was the winner or sole motivator. It merely may have been the straw that broke the camel's back.

The best frequency for your ad is multiple ads clumped together as closely as possible for that multiple impression hit. If you can only afford to buy a few issues, don't space them out over a long period. Clump them.

Tip #3. Recency targeting

Our research shows new to file names (i.e., people who signed up for that email newsletter in the past 60 days or less) are very much more likely to open and click than people who've been getting the newsletter for a longer period.

We also know that media company newsletters have the highest churn rates of any type of measured list. That means people are more likely to join, and then leave, a publisher's newsletter list than they are to do the same with a vendor-side newsletter.

In general, media companies are great at generating interest with an average monthly new opt-in rate of 6.9%, but less so at gaining long-term loyalty, with a monthly opt-out rate of 4.7%. (In comparison, a manufacturer's house newsletter gets a 6% opt-in rate but a .9% opt-out.)

This discrepancy may be due partially to frequency, but whatever the case, knowing the rules of recency you can take advantage of it.

First clump your ads over a tight period of time to get maximum impressions for the current readership. Then when you see response drifting off (as it will almost inevitably), pull back, wait a few months, and then launch another clump of ads.

The newsletter list may have had enough churn (people coming and going) in the meantime so that you're advertising essentially to new names and hard-core fans. Either way, they're your best prospects.

Plus, see if you can do an add-on media buy of an offer in the newsletter's automated welcome letter to new subscribers. These often-dull transactional mailings can get even better response rates than any ad you ever run in the newsletter itself.

Tip #4. Co-registration add-on

Next, ask if the media company would allow you to post an opt-in box on their sign-up form as a co-registration. Again, you're taking advantage of recency because these are the very newest names the publisher's got.

Co-registrations are also a great tester media buy for you if you're not sure whether to run a series of ads. Try 1,000-5,000 co-registration names and measure the results carefully. If these don't perform, it's unlikely your ad in that publisher's newsletter will. This is just the wrong list for you.

By the way, 32% of marketers we surveyed last fall said co-registration names performed as well as their house list generated from their own site. An additional 30% said co-registration names weren't quite as good as house names, but still well worth the investment.

Tip #5. Privileged positioning

This tip applies to all marketers placing ads in newsletters that are read by recipients from their at-work computers.

69% of businesspeople surveyed by EmailLabs last November said they always or frequently use the preview pane to look at their email messages. If those, 33% said they read all of the email in preview "even if I have to scroll." An additional 15% said they read "as much as I can without having to expand the pane or scroll."

What does this mean for your ad? Placement is more critical than you thought. A position that's likely to be at least partially inside the preview pane is going to get you far more impressions than a position elsewhere.

Key -- It's not just about being near the top. It's about being near the top left corner.

Quick creative tip: It's all about the text

The biggest mistake many marketers make with email ads is to repurpose graphical online ads -- aka banners.

Remember, email recipients are not in "surfing" mode. They're in reading mode. Except for very select situations (newsletters that are themselves collections of images), your best creative will almost invariably be text.

If you have to repurpose any online creative in the interests of saving time and resources, use your best performing search marketing ads instead of your best banner.

And, if you can, pop a keyword into the headline that matches the subject line of the issue. Yeah, it's tough as wording decisions are often made by editorial at the last minute, independent of ad content. But if you're a major sponsor, you can negotiate for help with this.

Otherwise, use keywords that tend to appear in subject lines from past issues. These are the words the readers are trained to respond to.

And, just as with your own email campaigns, shorter is better. You can educate, impress and convert on your landing page. Just use the ad to get the click.