I recommend that you use stamps, not signature fields. They are not the same thing.
You can add as many stamps to a document as you wish, without having to save along the way, but signatures are meant to be applied at the end, after the form is completed.
You can even design your own stamps and make them dynamic, meaning they add things like the user name and current date, etc.
Have a look at: http://www.pdfscripting.com/public/department53.cfm