There's one nice and very useful trick in there that could be considered as a half-cheat, as that's only an exploit of a bug in the game:
If you have some potion carrying around (healing, mana or time-stop potion - that one is mighty
), you can do the following to use it forever without getting away - throw it on the floor and cast some lightning or fireball spells on the potion, it should turn to ashes now. Get up this ashes again - they still have the same effect as the potion and you can use them FOREVER!
Just be sure to not throw the ashes somewhere on the screen, as they'll "broke" then and disappear.