I disagree with last hitting (*added this: Last hitting is the technique of hitting a creep just before it dies, without doing any previous damage to it) making or breaking an ap mid. AP champions tend to have problems last hitting, especially near tower(You don't want to miss creep kills due to your tower). So it works great for champions like morgana to push her lane with and area of effect ability like tormented soil.
In my opinion things like this are good to focus on (not in any particular order):
High CS: Kill as many minions as possible. Try to aim for about 200 cs by 20 minutes. You can maximize your cs by taking wraiths when your minions are by the enemy tower. When pushing to the enemy tower, you also don't want to stand up by their tower to poke the tower without safety from wards. Also, by pushing your lane, it gives you the chance to kill wraith camps/leave lane to gank side lanes.
Map Awareness: This is probably the most important if you want to live or die. You are in the center of the map with the least distance to travel to any lane. If a side lane has an opening for a gank, you should take advantage of it. If an enemy is missing from a side lane, you might be in danger, an ally might be in danger, or an objective is being taken. Wards can help you stay alive.
Counterpick: Some AP Mids work better against other AP Mids, use lolpro.com which is a website guide with proven counters from pro LoL players.
- Magic Penetration is the most popular choice for marks.
- AP per Level or Mana Regen per level is a popular choice for seals. (Some AP Mids take health per level)
- AP per level or Mana Regen per level is a popular choice for glyphs.
- Movement speed, ap flats, or ap per level is a popular choice for quintessences.
21/0/9 Fares well always, since you want to maximize your damage as ap mid.
Decisions that will break an AP mid:
- Giving first blood
- Not calling 'mia' or 'ss', not ganking your side lanes/assisting jungler.
- Not getting blue buff when you need it
- Low CS compared to your rival.
Items: (I obtained these item recommendations from observing professional play, summarizing guides from professional players):
Starter: Boots + 3 pots. This is the most effective to start with since with it, you can avoid getting ganked, you can chase, you can harass, and you have potions. It is more effective than doran's ring because low level skills don't benefit as much from the extra ap from doran's ring. The mana regen from doran's ring will probably get you enough mana for 2-5 spells before you buy again, but you might lose health because you can't escape trades with the enemy player.
Doran's Ring: Most commonly bought 1-2 times, usually not when a rod of ages is rushed.
Rabadon's Deathcap: The core ap item for any ap champion,
Will of the Ancients: A strong, rushed sustain item for Kennen, Morgana, Cassiopeia, Ahri, Vladimir, Swain, Malzahar and Viktor. Usully rushed after boots.
Rod of Ages: Strong, rushed health/ap item on Annie, Swain, Malzahar, Kassadin, Anivia, Morgana, Veigar, Zilean, Orianna, Galio, Gragas and Viktor. Gives a good amount of health and mana with a very fair 80 AP.
Zhonya's Hourglass: Strong item that has synergy with Morgana's ult and Kennen's ult that makes it worth rushing. Works well on all AP carries as a defensive item.