The proper way to have Javascript call PHP scripts

Hello,

I am currently working on a project that requires users to down vote and upvote things, an example being links in a post.

I have all of the voting data saved into custom fields for each post.
Currently when a person down votes or upvotes something I have a Javascript function that makes a xmlhttprequest to a PHP script somewhere on my server.

The script loads wordpress functions using
require_once('../../../../wp-load.php');

and I update_post_meta or do whatever I have to do.

I am wondering if this is the best way to go about getting this effect and what is the best practice for doing something like this?

Thanks