At the last GreenJUG meeting, we talked about how you should always use POST instead of GET for any secure web sites. But when you use a POST form on an HTTPS site, navigate away, and click the Back button (on Internet Explorer), you get "The web page you requested is no longer available - try refreshing..." It essentially breaks the back button which UI designers will tell you never to do.
So for me, the rule is, for any view-forms (where the form sends selection criteria for what the user wants to view), make them GET. Such forms tend to submit only the ID number of whatever they are pulling from the database, usually do not involve entering sensitive information, and should work when the users navigates away and uses the back-button. For update forms (where the user submits new data or changes old data) make them POST because it's good to prevent accidental resubmittion and because people tend to enter private and proprietary information on such forms.