That depends very much how your server is configured and what your intentions are. GET posts in the location-bar - so it could easily be modified - while POST does that "silently".
There are viable uses for both, although I personally prefer POST as I am all for keeping the location-bar clean
There's a limit to what you can send by GET as well which can make POST the only choice sometimes. Although I'll often use the two together by using the POST method in the form and attaching some variables to the query string of the action page.