see: http://codex.wordpress.org/Writing_a_Plugin#Updating_your_Plugin

update docs\Changes.txt
update date in docs\Changes.txt
update Upgrade Notice in docs\Changes.txt
update version number in docs\Postie.txt
update version number in postie.php comment

run deploy.cmd
commit
branch trunk to new version
add release post on http://postieplugin.com/
add release post on http://wordpress.org/support/plugin/postie