Changing visibility of controls in code behind runs after the rest of the sub has completed?

I have a couple of FileUpload fields and wanted to show a makeshift progress indicator while larger sized files are uploading. To do this I have the code behind page showing a panel when the upload button is pressed indicating that the files are being uploaded. The code to make the panel visible is the 1st line of the sub that runs with the OnClick event of the button and then the rest of the code (checking file type/size and then uploading etc..) is run. The problem is that the panel is shown after the file upload has completed no matter how long it takes to upload the file. I have even tried to do a thread.sleep(some value) to do some testing but still no use. 

Any ideas anyone?