Every time you build a new application in Drupal you *always* have to carefully govern the number of options available to normal site editors, or their heads will explode. (Literally. I've seen it.)

Almost every module you install will put something more on the node forms, until you end up scrolling forever just to reach the Save button. Not to mention most of this stuff you won't want them playing with anyway.

There are a number of decent resources helping you to make your first steps in to AHAH with Drupal 6.x. I won't try and create another start-to-finish tutorial. I just want to highlight a couple of existing resources and raise some specific issues I had (which I found to be undocumented) and pull out some fundamentals so they are more obvious.

Firstly, this page on Drupal.org is by far an away the most useful resource I found:
http://drupal.org/node/331941

Secondly, the Form API reference guide's section on #ahah is most handy too:

There is an issue with the Drupal Forms API in Drupal 5.x. It assumes there is one form on a page so it does not handle duplicate form IDs very well (read, at all!) - as such, if your forms are loaded in blocks or you just drupal_get_form() to load extra forms on a page in Drupal 5.x, you might well find yourself with several submit buttons all with an ID of id="edit-submit". Which sucks!

So you've got a client who wants to have a Drupal form send something off to another website. In my case my client wanted to manage their newsletter from the SalesForce CRM (which they doubtless paid gazillions of dollars for, so they'd better bloody well use it, etc.) so newsletter sign-ups needed directing there.