1. Make sure his spec is good, take a look at the basic Combat spec's and ensure his matches up. Slow mainhand and fast offhand, if its a fist make sure to respec for +5% crit.
2. Be smart with your combo points to maximize damage. If you have 4 combo points, and an enemy is going to die in 5 seconds - don't rupture, just eviscerate. There will always be situations where following the regular rules of rotations need to be altered.
3. Glyphs, make sure he has the right glyphs.
4. If you can, stealth and open with Garrote. But if you spend 15seconds positioning yourself so you can get behind the enemy, its a waste.
5. On trash mobs, where there is a group of 2 or more, pop Blade Fury, and Killing spree - and if you have any trinkets that give additional AP, use those too (do not bother popping trinkets that give additional haste at these times, that is a waste) - do NOT pop Adrenaline rush during a Killing spree, waste waste waste.
There are a lot of other things to consider, but the best advice to give when it comes to rotations, is use common sense and practice on dummies. And ALWAYS stand behind the enemy if you can.