I have been promoting affiliate programs for quite a while now, although this is not my main focus. Basically I love to write so I just keep doing that and make a few sales from stuff I'm advertising on my blogs and sites along the way. While I don't monitor my statistics really closely like some people do, I do try to keep an eye on them and I have noticed some trends when it comes to conversion rates.
Firstly, there's a much greater chance of conversions if visitors come via a specific search. They really trust Google, and if they're looking for something to buy that will definitely shape their search query. So, if you're ranking for these kinds of keyword phrases people can often arrive at your site pretty much already primed to purchase something you're advertising.
Then there's the geographic factor. People do seem to be more likely to buy products and services that are produced in their own country. Sometimes the reason is obvious, such as when they're looking for a dating site. Obviously they are more likely to join if they know that it's based in their country since there will probably be more local members signed up to it.
But sometimes this geographic influence is a little more subtle. Say you're promoting an ebook about a subject that in itself is not geo-specific. I think that prospective buyers tend to feel that if it's written by someone who lives in their country it will be more relevant to them somehow, and they will therefore be more likely to make the final decision to purchase it.
Also, I think that factors such as a sense of patriotism, and the desire to keep the money within the country increase the likelihood of conversions a little.
Then there's the power of your own recommendation. There's no doubt that if you write something positive about the product you're promoting, people will be more likely to buy it. This applies generally, and is an even more powerful factor if you have built up a reputation as an expert in your field.
That's why writing lots of thoughtful blog posts and articles is still financially worth it in the long run. You will eventually end up with an opinion that people will value when making purchasing decisions. That's definitely something you should utilize -- although of course you wouldn't want to abuse it by going crazy and recommending anything and everything (which would surely negate the effect in time anyway!).
Of course there are many other factors that can increase conversions. However, from my experience these seem to be among the most significant.