Yesterday I discussed how it's not a good idea to
rotate banners if you don't have a lot of traffic. Instead, you can try putting multiple ads in the space where a full ad would go. This will help you determine which advertisers work best with your site.
You don't want to go nuts including dozens of ads on your page, but you want to give the handful of ads you'd like to test an opportunity to be seen as much as possible. Since some of the spots in the grouping might do better, it's best if you can shuffle the placements within the group.
When you do this, you want to keep the larger ad size in mind and select the smaller ads so that they fit in the space of the larger ad as nicely as possible. That way if you want to switch to the larger ad, you won't have to make any major changes to your template.
You may not think you'll want to switch to the larger ad, but it's a good idea to keep it available as an option. Advertisers like the larger ad sizes (336x20, 728x90, 160x600, etc) and you may have an opportunity to display just one big ad if you get an offer. You may also find that one of the smaller ads does significantly better than the other smaller ads in the group, so you can experiment with running it at full size to see if you can improve the performance even more.
The reason the bigger ads are more sought after is that their size captures the users attention. What we're going to try to do is group together smaller ads close together so that they visually take up a similar amount of space. That will get the user's attention, and they will have an opportunity to review different ads. Within a couple thousand page views, you should have a good idea of which ads will get the highest click-through rate. After around 50k page views, you should start getting a good idea of which ads will have the best conversion rate.
This post covers some examples of common ad formats and how to break them up into groups of smaller ads. This should help you come up with other ideas for ad sizes you have on your site.