Some users have mentioned about DNN's multi layers of div's in the markup and the difficuly of using jQuery to select an element deep in the layer hierarchy. 

You can use the developer tools (F12)  and select any element in the page and use the 'copy css path' option in Firefox and Chrome (IE might have a similar option) and use it to easily construct the selector for jQuery.

Or the quick and dirty way is to name your element with a unique name you know won't be used but yourself and use a selector where id ends with your id. Example: a text input with id 'txtFirstName652' and use a jQuery selector like :$("input[id$='txtFirstName652']"). jQuery will find it anywhere in the page. Make sure it's not in a templated container like a gridview or repeater. Some might mention performance but we're talking about milliseconds which a human won't notice.