When your post is finished, click on the ‘Preview’ button. Highlight your article’s content, right-click on it and select ‘View selection source’ (probably different if you use something else than Firefox). Then simply copy and paste the HTML code in the ‘Summary’ text box which is under ‘Advanced options’, almost at the bottom of the page where you write/edit your article.

That way, you will force the summary tag in the RSS file to contain your article. Indeed, some Planet aggregator extracts the text from this tag rather than from content:encoded which WordPress uses for storing post.