There are a lot of things to remember about writing ads, and this document goes into only a few of the most important things that you'll need to know when writing ads for your sim.
The title of the tale can be as important as the whole story - Without a good, eye-grabbing subject for the ad, no one is even going to look at the ad. You need, in that short bit of text to generate enough interest to get them to look at the rest of it. Ship name? Fleet that it's in? Unimportant. irrelevant. You need something to spark the imagination of the reader, not bore them from the start.
You need a "hook" to draw them in. - Just like the subject, you need to highlight something about your ship that is different than the hundreds of other games out there. If you don't have something different, unique to your game, then you need to find that one thing about your game that makes/made the people playing it choose your game over all those others. As an example I'll use the last ship that I ran. It was a horror-star trek sim. Rather than Cardassians, Romulans, whatever, my crew faced the creatures of nightmare. For you it would be a specialty of your ship, or of the dynamics between your crew, or something else that makes your ship different. Something that will draw people into your game and stay with it.
Short and Sweet - People these days have a limited attention span, they have trouble concentrating beyond the 30 seconds that it takes to show an ad on TV. Rather than a long explanation that tells everything about the sim, you need to boil it down to as little as possible, enough to get them interested and wanting to know more.
Above the fold/Below the fold - This is a key marketing concept from the real world that applies to any kind of advertising. Picture a piece of paper that is tri-folded. Your ad has to be most effective "Above the fold". In this case, it means that you need to put in the link to your sim at the very top of the ad. This way, if your subject grabbed them, but they didn't feel like reading through the entire ad, they still might click on the link and take a look at your site. By the same reasoning, you need to repeat that link at the end of the ad as well, so that if they read through the entire ad, they don't have to take the time to scroll back up to find the link. Make it as easy as possible for them to get to your ship as possible. Don't use a fancy link either, keep it to the main page of your sim, don't even add on the /index.php if you can help it, it's unprofessional and clumsy.
A picture can be worth a thousand words - A picture can make an ad stand out, if it's good and related to your ship specifically. A poor quality picture (resolution or too obviously photoshopped) can hurt you more badly than not having a picture at all. Having a generic picture of your class of ship that isn't specifically for your ship, one that's widely available? Just as bad. You also have to remember WHERE you are advertising. Yahoo groups allows people to choose text only, 90% of them will never see the pretty picture and the ad blaster only supports text ads. On forums or other places where you can use the pictures, go for it.
While this this isn't neccessarily about the ad, it's also important. A lot of COs use "basic, boring, out-of-the-box" Nova. No special skins, no special made graphics, nothing different. While this won't absolutely kill you, you have to remember that if you have things that are different, those differences will interest people and make them take notice, they make your sim stand out from the rest.
There are many people who develop Skins and Graphics, some of them can be contacted through the Images section of the Forums.