[jQuery] Selecting form ID from $(this)

Howdy Guys,
A bit of a problem I have been struggling with for a number of hours,
and some help would go a long way.

Its not really a problem with the $.ajax() function. It is more a
problem with selecting live variables on the fly. I have multiple
forms on one page, so selecting that one form to use and update data
is what I am looking at doing.

$.variables = {}; //Sets up an array so I can call the vars in which
ever function I'd like to.

$('[id^=credit]').change(function() {
$.variables = {formID : $(this).attr('id')}; // Meant to pass the
id, but passes an object.
$.ajax({
type: "POST",
url: "/credits/edit",
data: $(this).serialize(),
beforeSend: function(){
$("#[$.variables.formID]").append("YAY"); //Just to put
something in this form id.
console.log($( [$.variables.formID] )); // Returns this in the
console: Object length=1 0=credit1 jquery=1.3.1
},
success: function(html) {},
})

So I have set the variable.formID after the .change function. It
passes an object, but I am looking at passing the actual id, so that I
can put some data into that form id...

If I use this $("#[$.variables.formID][0]").append("YAY"); code in the
first line of the before send (in an effort to get to the [0] in the
object it passes, it updates all forms on the page with the .append.
But it only sends the data from that one form.

Because there is a number of forms on this one page, I'd just like to
be able to select the id for the form, so I can update data in that
form - At this stage I can get data into any form with an ID, but not
the form that has been used.

Am I missing something? I could have completely stuffed up how to do
it. Any tips would be welcome.

Thanks