Usually, one uses Get for retrieving things, so that the users can easily bookmark the retrieved page,
and Post whenever something is submited, like forum posts or wiki pages.
Additional Forces:
-Get is limited to 1024 characters, posts can be much larger (but it's limited at the server side, at 2mb for PHP by default, for example)
-Post is a tad bit more difficult to mess with. But only one tad. No more.
-Get-Requests can be easily cached by the browser or proxies.