POST vars are more secure because they are not visible in the URL. Also, if you are posting large amounts of text, GET method is not going to work well. I generally only use GET (querystring) method for pages such as a product detail page. I might do something like product_detail.php?id=15. In this case, I like having the ability to directly call the URL for a specific product. Also, I'm very likely to be using the querystring URL in a link. You can't POST in a link. e.g. <a href="product_detail.php?id=15">Daniweb Logo T-Shirt</a>
As POST is more secure than GET, and can handle larger amount of text, why we still need GET? Even if we cannot POST in a link like <a href="product_detail.php?id=15">, we still can use POST in form to pass id, right?
General rule of thumb: Use get for pages that views stuff and POST for pages that change stuff. For instance, you don't want an URL a la "www.somedomain.com/products/raiseprice.php?rate=10%", because if someone calls this page twice, they will change the price twice.