There seems to be a difference of opinion on whether parking a ship in orbit over a non-affiliated planet can make a distance... I can assure you that if a planet has basically evenly divided loyalty (which many do) it can make a difference. I have done it several times using nothing more than a corvette. It takes a while (about 100 turns) but if you have an extra ship in an area, and no available diplomats close by it is an option.