Have at least 16 trees in each acre(except the northern and southern most ones). To make acres, set 16x16 block patterns all through out town. If you do it right, they will cover the whole town without overlapping or falling short. Again, plant at least 16 trees in each one(planting them on the border counts as in that acre), and don't forget, flowers also compensate for lack of trees.
A guide would do you better justice, though. Or I can always help and come to your town. PM me if you want.