Always start with Dorans Ring. The health and mana regeneration boosts are great for early game, not to mention the extra AP.

Typically I mid as Morgana, so it will be awhile before I can go back to buy items, given I'm against a good opponent. So usually by the time I go back, I can outright buy Morello's Evil Tome and Boots of Speed. Morgana has some great burst abilities, but they have a good amount of cooldown, ~10seconds for some of them. That's not too bad, until late game when you can't kill people just by blowing all of your abilities and a few auto attacks. And forget team fights in extreme late game. So CDR is a must for Morgana. You trade some AP, but, you'll be more useful to your team in the end.

At this point I go ahead and rush Rod of Ages. You'll want to get it as soon as possible so you get the full use of the buffs and stacks that it give. Any good spell caster uses Rod of Ages.

Around now you have ~170AP. I'm usually behind by a bit in AP at this point compared to other spell casters, but the other items are really needed to keep Morgana playable early game. She's mana hungry and has long cool downs. But now is the time to start that Rabadon's Death Cap, when you finish you'll be over 400 AP.

Ionian Boots of Lucidity. I thought about these for awhile and went back and forward between Sorcerer's Boots and the Ionian Boots. But in the end, CDR is better for Morgana, especially since there are other items that can give you Magic Penetration.

Zhonia's Hourglass is a great item and it is actually amazing for team fights. Run in blow your ultimate and shield with a flash. Then use Zhonia's and you will probably survive till the end of the team fight to be helpful.

So at this point, I either choose between Abyssal Scepter and Void Staff. It really depends on what the other tea is building. But usually games don't last this long unless they are really good.

Clarity give you that early game staying ability and helps some in the mid and late game depending on your team mates.

Flash is a great initiator thru out the whole game. I love it.