Autocomplete for @username mentions

Being able to mention someone is great for encouraging conversation, but right now it's a little cumbersome to do so. Either you copy the username, which is not easy because it is a link and you can accidentally click on it, taking you to the profile page, or you write the username manually and you can mistype it. It would be great to have autocomplete for @usernames mentions. ps. Does anyone have a special trick to mention other people?