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).