I think probably the input names in your HTML aren't correct or aren't being generated correctly (if you're using the form helper). Double check your HTML to be sure (view source in a browser).
Data stored in Controller::data variable comes from POST values where the input's name starts with data. So this input tag will have the value in $this->data['Anything']['Something']:
<input type="text" name="data[Anything][Something]" />
When you use the form helper, and do this:
<?php echo $form->input( 'Something' ); ?>
The form helper is smart enough to know which model you're using (I think because it sets an attribute when you call $form->create('Modelname')). So the above (PHP/form helper) example would output:
<input type="text" name="data[Modelname][Something]" ... />
Of course, there are a few caveats (Modelname.Something must be a field that exists in the corresponding database table), but you can learn more by looking at the manual.
And finally, I believe the $this->params['form'] attribute has all the POSTed values, regardless of whether you prefix the input name with "data".