How can I make it so that when someone gets an error while registering an account on my site and clicks the 'back' button to fix an error, the values they previously had are still present?

I thought they did so automatically, but... I guess not?

One way is to use ASP.NET - it automatically maintains the viewstate, and handles client and server side validation very easily.

www.asp.net

...or you could just use javascript to do a form validation. so when the user puts the wrong info in, the user is noted (alerted) before they even submit the form.

If possible, you should put server side validation in as well. Javascript validation can be circumvented very easily, so if you're validating for malicious code, or something that could have an adverse effect on your application, you should always implement server side validation.

Well, it's all handled by PHP right now. The security factor is pretty low because I haven't implemented all of the required checks yet, so on and so forth...

In all the form values, give each the corresponding POST values. For eg, for a form element named "username", give the element a value="<?php if(isset($_POST['username'])){ echo $_POST['username']; } ?>"

Also at the top of the page, include:

header("Cache-control: private");

This will help stop IE from clearing the cache. Be aware of javascript validation for forms that do database insert queries. In some cases the header is sent twice which results in a double insert; a very, very annoying bug which happens with all browsers.