I think the size of the grind has something to do with it. I've ground my burger extra fine before and it had trouble holding together. I think a medium grind works well.
In terms of adding anything to the burger, I've done the pork thing but gave it up. I now add about 5-7% quality Beef Suet (available at most meat counters). It is easy to figure the approximate amount. Just go a little under 10% and you'll be fine. I find that such a small amount of Beef Suet doesn't change the flavor very much, but adds just a little moisture to the cooking. The small fat golbs also hold the patties together when forming. I think it also lasts longer in the freezer than pork.