You'll want to start your fights with Scorch. This will put Critical Mass on the target, which increases your chance to crit by 5%. Next, throw out your Flame Orb. Followed by Living Bomb. Keep both Living Bomb and Critical Mass up on the target at all times. (Remember: Pyroblast! will also put Critical Mass on your target) In an ideal situation, you would then spam Fireball until the end of eternity. However, you'll find yourself going OOM fast... very very fast. This is where Scorch Weaving comes in.
Scorch Weaving: This is the reason we spec 2/2 into Improved Scorch. Our Scorches cost us 0(!) mana. It makes Scorch our most mana efficient spell by far. However, Fireball does more DPS. In order to keep our mana pools at a constant level, we must Scorch Weave. That is alternating between Scorch and Fireball. Therefore, mages do something like this, during a fight:
Fireball to 80ish % mana, Mana Gem, Fireball to 30% mana, Evocate, Fireball to 50% mana, Scorch Weave. All of this while keeping Living Bomb and Critical Mass on the target and using Combustion (see below for more details on Combustion)
Keeping your mana at 50% allows you to burn down the boss with Fireball if necessary. This could happen if a boss has a mechanic increasing it's damage taken or if an add needs to be burned.
However, keep this in mind: You do NOT want mana left over during the fight. Left over mana is wasted DPS, since you could have (possibly) casted one more fireball instead of a scorch.
Combustion: Finally, I'll talk about Combustion. Combustion is the Fire mages 2 minute cooldown. It has a very interesting mechanic. Let me explain it with an example:
Example:
Your Living Bomb is ticking for 300 ever 3 seconds on the target. =100 DPS
You just got a Pyroblast! You cast it on the target and its DoT is ticking for 75 every 3 seconds. = 25 DPS
The Pyroblast crit, and the Ignite from the crit is ticking for 150 every 2 seconds. =75 DPS
This would be the ideal time to use Combustion. The Combustion would tick for 200 damage per second (Please Note: these numbers are completely made up)