How to change tweet format

I was able to successfully change the way notifications of new blog posts are displayed on twitter by modifying this line of code from

$this->tweet_prefix = 'New blog post';

to

$this->tweet_prefix = 'New story';

I am trying, however, to change the tweet format more significantly. I would like to display the actual post text (most of which are short) or at least the first n characters of the text, with a link to the rest. I would also like to include the author’s name in the tweet.

Thanks for any help.