I have two selects:
All of these methods are great. I have found another simple resource that is a great example of creating a dynamic form using "onchange" with AJAX.
http://www.w3schools.com/php/php_ajax_database.asp
I simply modified the text table output to anther select dropdown populated based on the selection of the first drop down. For my application a user will select a state then the second dropdown will be populated with the cities for the selected state. Much like the JSON example above but with php and mysql.