It's true that on average users a lazy, egoistic and selfish bunch but you need to educate them to make them work. One simple idea (pretty obvious for any experienced social network creature) that seems to capture their minds quickly is: the more you tag your posts the more internal links to your post will appear from within the site thus increasing your post popularity in terms of PR, its searchability, findability etc etc.

Making users to tag someone else's items is a bit more difficult but look how MyBlogLog solved this by providing Tagged By and What Was Tagged MouseOvers