You need to store your nectar in a cellar for maximum increase in value. It does not matter how deep the cellar is. If you build your house on a foundation your first level down is a basement. You will need to build another basement below this, a cellar. If you build directly on the ground then you will build a cellar immediatly below your ground (USA first) floor. You do not need to go any further down uless you want the extra storage space.
Incidently, buy the most expensive nectar rack, (the necteaux 1,200 simoleons). It will make a significant difference.
The better the quality of the fruit used, the better the nectar.
Home made nectar improves significantly faster than purchased or "found" nectar.
The best nectar (I have found for value increase) uses 9 lifefruit and 1 pomegranate. Others may know of a better recipe.
If you go on vacation, take your nectar with you, in it's rack, in your family inventory. If you have a vacation home place it in the cellar when you arrive, othwise, it will still continue to age (though more slowly) in your inventory. If you leave it at home, it will not age.