The best way to keep herb bushes alive is to have them grow in 2x2 patches. New bushes will not emerge from this, but the old ones won't die (as long as you don't over pick them).
Herb bushes have three life stages:
Full of Blosoms - This is as healthy as a bush gets
Strong - This bush is steady, and can be picked safely
Withered - The bush is about to die, do not pick any more herbs off
This is a list of herbs and effects:
(Remeber, cursed herbs will always be bad, never eat or use them, and blessed herbs will always have a better effect than normal)
Stomafillia - Fills your stomach
Stomacemptia - Empties your stomach
Pepper Petal - Heals you somewhat
Spenseweed - Heals you more
Burb Root - Damages you
Alraunia Antidote - Helps overcome poison
Demon Daisies - Poisonous
Curaria Mancox - Fights sickness
Devil's Rose - Causes sickness
Moss of Mareilon - Excercises Dexterity
Morgia Root - Excercises Willpower and Toughness