What are your most important hints to improve the CTR on organic search results?

Rank HIGHER. ;)

If you're asking how to get more clicks from a #1 or #2 ranking, I would try these:

- Make sure the Title tag is readable
- Make sure the Title tag contains action items and selling copy.
- Make sure the Title tag contains the target query
- Make sure the URL contains the target query
- If the snippet displayed is using the Meta Description, make sure it contains the target query but also sells the user on the page. Does it describe the content of the page? Does it contain any action items?

It's also important to mention that you might not want to toy with the Title or other tags too much as you don't want to disrupt the rankings.