It seems to be luck as much as anything when a villager reaches 90. I don't think the player can do anything special to influence it.
Playing the Nature faction should result in the villagers living longer, but on the few occasions that I've got the award I've played Science as often as Nature.
Regarding the potions, as mentioned already, they must all be different. The order in which a potion is mixed makes no difference. Therefore a potion consisting of herbs x, y, and z counts as the same as one made from y, x, and z, or z, x, and y, and so on.
I can't remember the exact figure now, but I seem to remember that there are 56 possible combinations of herbs to make the potions; just over 50, anyway.