Depends what you want to transmit, if you just want to tell the webserver which page of a search you want then you might use script.php?page=1 however if on page one you had to login it wouldn't be a good idea to have the username and password in the url and so you would use the post method to hide these variables, generally any data that is large (+5 chars) or sensitive at any level you would post, anything else it's safe to use get.
On both methods, it's always a good idea to sanitize the characters with stripslashes(trim($_POST['var'])) <-- same for get, also if the data is ever used for a mysql_query statement then make sure you use mysql_real_escape_strim( stripslashes( trim( $var ) ) ) or similar, look it up...
Also, try to check for instance if you expect a form to post numers, that the posted variable is numbers, or vice versa..