This will do the trick!
add_action('trash_post','my_trash_post_function',1,1);
function my_trash_post_function($post_id){
if(!did_action('trash_post')){
// do stuff
}
}
Here we add the function, and to prevent the hook from executing more than once using did_action:
http://codex.wordpress.org/Function_Reference/did_action
As always, these kinds of hooks take the form {post_status}_{post_type}
, so trash_post
, trash_page
, trash_customposttype
, publish_page
etc