Start off with a few (sometimes 1 if heavy melee DPS in group) Scorch... for the +Dmg effect but also just for the fact that it wont cause any sort of agro problems. Past that just Fireball spam...
Main thing to avoid is just being a complete momo and doing things like pulling with Pyro or using a Fireball Fireblast quickly too early in the battle.
Use Combustion when appropriate (it's not long cooldown, so not using it on trash every once in a while is a waste.) Try to use it when you either got some CC going on multies or actually tanking multies... when Combustion is active stick to Fireball/Fireblast since Scorch wont make it worth it.
Past that, remember that just cuz you're specced for fire doesn't mean you're restrained to only Fire. Frost Nova, Cone of Cold, Sheep, Invis, Manashield and many more are all still important Mage spells.