When I first played Nasus I noticed that early game I ran out of mana so quickly. I like to poke at the enemy with Spirit Fire. This makes them keep there distance, while I siphon strike the hell out of their minions while they stand and watch (works best when you are up against melee). Tear of the Goddess grants you 4+ mana when you use any spell, therefore you can spam my spirit fire like crazy. This will also allow you to siphon strike no health minions (better chance at last hit). You will never run out of mana