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1. The normal way is to use the <!--more--> tag to differentiate excerpts from full posts. Only the bit before the "more" will show on the front page, category pages, etc.
2. You can amp this up with the Excerpt Editor Plugin [wordpress.org]
3. If you don't care about where they get cut, you can limit snippets to a set number of characters with the WP Limit Posts plugin [wordpress.org]
Any time you are limiting post length, make sure to check your RSS feed and any email subscription options you have. I've had it in one case where the part after the "more" was getting sent and the beginning of the post not any time I added a "more" break.