Dynavistics has a nice article on Collections Before the Due Date covering the kinds of things you should be doing before the invoice goes out to make it easier to get paid. They also have what is shaping up to be a pretty nice looking collections module in Collect-IT.
Collections Before the Due Date – Dynavistics
Archived post from 2013. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.