There's an advanced option to control this.  If you go into Journal Setup, expand Advanced, and look for this option:

When I don't write a summary for a post:
Include the full post in the RSS feed
Include only the first few sentences

If you choose the second option, an abstract of the first few sentences will be used instead of the full post, not just in the RSS feed, but also on the main channel page. (I'll fix that option so that it makes this clear -- right now it sound like it's just for the RSS feed.)

You can also, if you prefer, on a post-by-post basis, write your own summary, rather than just letting us chop out the first few sentences. To do this, while you're editing a post, expand the Advanced section and choose Yes for Include a Summary. Note that summaries are currently in plain text, while the full post allows formatting, images, etc.

We'll probably get a bit more flexible about this in the future (for example, allowing you to control what's used in the RSS feed independently from what's used in the main channel page), but for now, that's how you'd do it.