So lets start with a disclaimer, this is my preferred arms rotation others may have a different view on what to do but this is what works for me. I will break this down into an easy to understand format as opposed to the single string format that is commonly used.
Open: Charge > Rend > Colossus Smash > Bladestorm
Main: Colossus Smash > Mortal Strike > Overpower (if available) > Bladestorm (if needed save for AOE parts of fights) > (Heroic Strike > Slam)*
*Only if MS is on CD and no Overpower available
20%: Colossus Smash > Mortal Strike > Overpower > Execute > Heroic Strike
Through out the fight I am watching rage and using Battle Shout and Deadly Calm to increase rage. I also open with Bladestorm because it allows me to generate rage quickly at the start while doing decent DPS.
I also use my on-use DPS items like trinkets and profession items throughout the fight and try to keep them on cool down.
With this rotation and ilvl 359 gear (with hit and exp capped) you should be able to easily push 15-16k DPS and if you are really good with the rotation 17k now as you get better gear it easy to push 18-19k and even hit 21k on some fights.
Now if you need help with timing there are a few different routes you can take. Lets start with the most important thing for increasing DPS through timing GET THE LOWEST POSSIBLE LATENCY, high latency will decrease your DPS because it screws with your timing. Next download Snowfall Keypress, this addon makes it so spelss are cast when the you press the bound key on your keyboard instead of when it is released (edit: I guess Blizzard added this to the game in 4.1, guess I should read all the patch notes).
The final addon that can help with timing and even for those of you that think you know when to cast heroic strike and when not to with the naked eye this addon will help, is SkeenCore3 and SkeeCore3ArmsWarrior. I haven't spent much time with this add-on but recently downloaded it and am glad I did. It reminds me of clcret which I used heavily for my Retribution Paladin in Wrath. It lets you know when things like Bladestorm and Deadly Calm are off CD and tells what spell you should use next. As I said before I havent spent much time with the add-on so I haven't found ways to set it up for my rotation (main and 20%) but it makes a great guide for the Main rotation.
Most of all a good rotation and timing comes down to you and your attention to the fight and knowing when you'll need a little more AOE or that rage is going to be hard to maintain in a certain phase. Just keep you heads up and don'f face-roll like a Fury warrior or you'll end up doing the DPS of a Shadow Priest.