Paying attention to last hitting with Siphoning Strike will have a large impact at the end of a game.
If you're solo, Spirit Fire is a great way to farm a lane. It can be bad if you're in a lane with 2 and you push too far. Find the right balance between Siphoning Strike last hits and AoE farming.
If you have low defenses, people will focus you down even during your ultimate. Try buying some survivability items even in your DPS builds.
While transformed by his ultimate, Nasus is stronger than most champions in the League. Engage him only if you have a clear advantage.
Max rank Wither is a very effective counter to Attack Damage characters, so try to avoid getting caught alone by it.
Nasus is prone to being kited. Try not to engage him at full Health.