I've complained about ads and store displays before and I've decided to put my money where my mouth is.
Here's an ad, for a a gown that I very much like from Crimson Shadow, that could use a little bit of work. The gown and ad are located at
Crimson Shadow Rezzable 229,166,27
Now, that ad isn't as horrible as some I've seen, but it could use a little work. In my opinion the ads major offenses are bad lighting of the avatar, and the text is placed directly over the background causing legibility problems.
Here's my quick and dirty remake of the ad, which still sucks, but is somewhat improved.

Notice how I have my avatar lit a touch better, though I probably should have turned up the contrast a bit on the avatar images compared with background. I've also placed the text, not over the background itself, but over a plain black "information area". Using such areas, separate from the item images themselves increases legibility, though I should have made the text bigger. My ad also shows the included shoes. Since I have that nice space at the side, I could add a store/brand logo which would have been an excellent idea. I might, depending on legibility, even be able to list information on the model's skin/eyes/hair right on the ad/display.
So there you have it, my own example of an ad improvement.