How do I restore mana?
You can also restore mana by spending 1 turn in a castle/town that you own that has a mage guild built in it. It will restore the entire mana bar.
There are items:
Charm of Mana - Hero recovers 1 extra spell point per day
Talisman of Mana - Hero recovers 2 extra spell points per day
Mystic Orb of Mana - Hero recovers 3 extra spell points per day
A secondary skill (Mysticism) also boosts daily mana regeneration rate.
Dungeon town has the "Mana vortex" building, it allows to instantly set your hero's spell points to 2*maximum (once in a week). If you are going to visit the town anyway, then it's basically free (no need to end turn in a town).