Topic. Besides the usual spellcating perks (conjuration, destruction, restoration, alteration...)

Not necessarily perks needed, but for skills, Archery and Alchemy are very helpful. It really depends on your playstyle, though.

Illusion is fast becoming one of my fave skills. Its nice for all spells also as it has the silent cast perk.

It's not just for mages but I like getting speech up to at least 50 so I can take the perk that allows you to sell any type of item to any shopkeeper. It cuts down on my obsessive need to sell what I kill running around time.

IT depends on your playstyle really.

Aside from Spellcasting perks, there aren't any perks that really benefit spellcasters in particular. Alchemy really helps, I just took all 5 "Alchemist" perks and the 2 alchemy perks that enhance the power of positive potions. Those powerful healing and magicka potions REALLLLY help.

IMO, if you use Destruction: Dual-casting and Impact are almost required. Damage perks (Augmented Flames/Frost/Shock) really help too.

If you're playing a pure mage with no armor, then in Alteration, the Mage Armor perks will help a lot. Also in Alteration, the Stability perk is nice to increase duration of all alterations. The Atronach perk in Alteration is REALLY helpful too.

In Restoration, the Regeneration perk is extremely helpful, and Respite is nice if you also use weapons. Ward Absorb is nice, but not quite as nice as I was hoping it would be. Still helps out a bit nonetheless.

Conjuration is one of the most helpful schools, and IMO if you use it you should probably focus on eithe Atronachs or Undead, go up through that line and eventually get Twin Souls. Very, very helpful.

If you're going to use illusion at all, get the Illusion Dual-Casting feat. Really helps empower those spells to affect more enemies. If you want to keep using illusion at high levels, then get at least Animage and Kindred Mage. Oh, and the Illusion perk Quiet Casting is pure gold (one of my faves) because it works for all spells of all schools. With this perk, no enemies will hear your spells and become alerted to your presence (but you still hear your own spells). EXTREMELY useful.

The spell resistance and absorption perks in aleration are must haves, but take a while to get to.

Early on you should get damage boosting perks in Destruction, and faster magicka regeneration in restoration, and then some conjuration.

I prefer to pick one damage line of perks only (either fire, ice or shock), and either atronachs or undead perks in conjuration (I went with fire + atronachs).

After Destruction / Restoration / Conjuration are perked this much, you can start perking alteration / illusion. You also want some parts of Alchemy (better bonuses for potions you make), and some of enchantment (middle line that makes HP / Magicka / Skill enchants more powerful, you dont need weapon damage perks on a mage).

If you want to wear armor as well, then you want to go around the right hand side of smithing up to daedric armor.

It sounds like a lot of perks, but what you need to remember is that you never have to fully perk each tree that you use. On my mage I want to use armor, so I decided to drop illusion and use destruction / restoration / conjuration / alteraton (skipping mage armor, taking magic resistance and absorption) / alchemy / enchant / smithing.