A while back I did a day of consulting for James Wang on his SQL Pretty Printer software. One of my many recommendations was that he displayed the name of the licensed user prominently in the software, to discourage casual license key sharing. He recently reported back to me that he had made this change and it had significantly increased the number of orders for multiple licenses. The average number of licenses per sale increased from 1.34 to 2.65 since he made the change in February 2011. That is a 98% increase in sales! And it only took a week to change the licensing system.
Each time I have had an increase in sales, I found that it was mostly due to marketing related issues like changing the licensing system or paying for a link from a site which has a highly targeted audience for my software, not due to a new version release.
Of course it helps that James had a good product in the first place.