Everybody knows that Akali is a champion of balance. I hold true to that term as early game as possible by first splitting up her runes and mastery. The idea is to build her while activating both her disciplines with extra early game movement speed. Also to give her a little defense and health regen. All of this while still giving her maximum damage potential.

Mastery's,
12/3/15

Runes,
6 marks of strength, 3 marks of insight
7 seals of resilience, 2 seals of strength
9 glyhs of potency
1 quint of swiftness, 2 quints of potency

This exact pregame build will allow maximum move speed while still maintaining twin discipline activation. Oh, and the 3 marks of insight are there because of the room that was created due to the revamping of the mastery's. A little magic pen goes a long way. XD

The mastery's are set up to give max move speed as well. Put points in first to insure that you have 4 in swiftness. Then finish the utility branch to use 3 points in transmutation for the spell vamp and 4 more points in where you may please. I have mine in greed because I enjoy a little extra gold.

For the defense branch, just put 3 points in resistance. You will have all the armor you need with the armor seals and doran's shield.

Finish off by having 3 points in brute force and 1 point in mental force. This last step is required to be able to activate your disciplines at the start of the game. Put the rest of your points in to finish arcane knowledge and 3 points in for havoc. Just for that extra damage ratio.

I suggest starting off with a doran's shield by giving you the best lane survivability very early on. The point of adjusting this build for more move speed is to start off with this item, maximizing your ability to escape and chase in your lane. Also the doran's Shield's health regen combos well with the spell vamp of your twin disciplines and transmutation mastery.

If you happen to be doing very well in lane, get your hextech revolver as soon as you can. Continue getting damage items to build into either a hextech revolver or rylai's crystal scepter. If luck is running poorly then just settle for lvl 1 boots and work on getting better items later. But the goal here still is to get the hextech revolver. That is your main survivability item. If you're still getting harassed in lane, get giants belt then build into rylai's crystal scepter. Finish the gun blade later. From there, build into either lich bane or banshees veil. Void staff is also one of her best items. Get that one wherever you see fit.
Depending on how you feel about movement speed, get either sorcerer's shoes or mercury's treads.
One last thing is if your items are filling up. Selling your doran's shield later game will be useful.

Pros
Strong tower diver
fast bursty damage
strong escapee
strong lane survivability
anti-carry

Cons
not a team carry
still pretty squishy
easily built against
typically focused by enemy team
vision wards, nuff sed.

While in lane, harass with mark of the assassin(Q) as much as possible. And if can get close enough consume the mark for more damage. If you think you can't harass without taking too much damage then just farm using her Q and keep your distance. Once lvl 6 is reached and you have your ult, then begin countering with her combos.
There are several different ability combos you can use to harass/kill a champion.
First combo,
First throw down twilight shroud(W) on the champion. Quickly ult(R) then Q them, consume the mark and then use crescent slash(E).
Second combo,
First ult, then Q, consume the mark, crescent slash and quickly throw down your shroud for a quick escape.
Her third and strongest combo,
Get close to throw her q on the champion. Allow your Q's cooldown to reset, then quickly ult to consume the mark, throw another Q on them and consume that mark and crescent slash, and shroud to get away.
Performing each of these combos successfully will garrentee lane dominance and kills.

Using whatever summoner spells is up to you. I myself use Flash and Ignite on Akali. I use Flash for last second escape attempts or get myself closer to my target. It is also helpful tool to jump over walls. Ignite is a fantastic spell to garrentee kills. Use it champions to nerf their healing ability to give you the edge to win a fight or use it as a last chance to get a kill. If the champion happens to get away with really low health, throw on ignite to get the kill and get away faster. These two spells are excellent tower diving spells.

I don't use a spell like ghost for chasing purposes because once rylai's crystal scepter is attained, there won't be no need for a spell like ghost.

Heal could be necessary to change the outcome of a fight by suprisiing a gank attempt giving your team or lane a second chance to win a fight.

Smite is only necessary for jungling and Akali is not a true jungler.

Clarity, clairvoyance, promote, garrison, and fortify are spells that don't benefit Akali whatsoever.

Teleport, rally, cleanse, and surge only benefit what type of play style you like to use.

And revive is the most useless spell of them all.

Despite Akali getting nerfed, she is still a viable champion whether she is fed or used correctly. Akali is and probably always will be my main champion. I've played her enough to adjust her build to her full potential. In fact right now I am going to work on her rune page some more and figure out how I could fit a spell vamp quint in there. It is a work in progress and I think I can still make her better. Good luck and have fun with all your matches and I hoped you like my guide. I'll see you on the battlefield.