Its quite easy to provide a link that creates a twitter status update for something you are promoting in your blog.  Same for Facebook.   You dont want them to just go to Twitter but you want to help them out by filling out their status promotion for them.  Usually in your blog software you can highlight some text and select the link icon which will pop up a small window and allow you to insert a target url.  All the information is in the target URL.  
 

In order to understand whats happening you need to know a little bit about how parameters are passed to a URL.  Parameters are passed in key=value pairs.  The first parameter is preceeded with "?" and subsequent parameters start with "&".   I know its geeky - but little technical knowledge can help you understand whats going on.

For Twitter we just specify the http://twitter.com/home directory and specify the key=value pair of "status=what you wanna say and which links you wanna specify"

For Facebook we do something similar when "u" is the url parameter and "t" is the text parameter.  These parameters are passed to the PHP method http://www.facebook.com/share.php.  One thing I have noticed is that the title field gets overridden if you have a meta title (html title) set in your page.  Someone might want to verify that.