For instance health and wellness, or if possible a less general theme and more of a niche business, high demand and low supply. This will be based on your business concept. Or if you are doing Adsense sites your highest paid keyword.

Put good optimized content relating to this topic:  Make sure you have plenty of good high quality content. This should be highly relevant to your business and directed specifically to your market.

These can be used for tracking your advertising. Remember to put good informative content in them. For example putting information about the different products your affiliate program offers. You can also write or have written unbiased product reviews. People love good book reviews, these will be seen as good and relevant resources for your visitors.

Remember to interlink and if possible to optimize these landing pages, so that Googlebots can crawl and get information about your content to add to your website data.

Of course there are always some mistakes to avoid: The most common one is joining all sorts of affiliate programs that are not necessarily related to your theme or business. If you do this you will bring down the quality of your site and make it look like a swap meet. This will definitely work against you with Google, as they will see this as not having relevant links and resources for their customers (your visitors). Besides this you will get all sorts of visitors who are completely unrelated and not targeted to your business so you are unlikely to make many sales.
 
Do not launch too many websites at once: Whatever you do if you are making affiliate mini sites or Adsense sites do not launch too many at a time. Google will view this as trying to spam them and either throw your websites into the sand box or penalize your Page Rank. Slow and sure does the trick.
Don’t link up your sites that are unrelated: again Google will think you are trying to spam the engines with unrelated linking and you will get penalized