I find it beneficial to send a "Have you Overlooked This" stamped invoice right at 30 days - that gives them immediate notice and the chance of going over 45 is next to nothing.  If it is not received that next week another one is sent weekly until paid.  It also tells them in an inadvertent way that you keep up with your accounting and expect the same attention to your invoices that they do to your work.  Utilizing this method - I have had a very rare occassions to call.  Even the best of my clients have missed an invoice here and there - we are all human and leave things in places we shouldn't have - like on your desk instead of the accounting dept!