Everyone has a method for handling forms but it usually gets complicated when there are file uploads in that form.
Fine Uploader is a plug 'n' play script that ease file uploads and improve usability with the Ajaxed interface.
The script was known as valums-file-uploader (was featured at WRD) and deserves a 2nd shot with the improvements it had.
Uploader can be the standard "file input" or it has support for drag 'n' drop uploads as well.
Also, there are options to limit the file size, file types, number of files and more.