It's important to keep a close eye on your heat gauge whenever casting, with the exception of The Equalizer. All of Champion abilities are stronger from 50% heat to 99% heat, but overheating (at 100% heat) will silence you.
Spam Flamespitter at the beginning of the game to get in the danger zone, increasing your damage.
The bonus damage on Junkyard Titan can be just enough for the kill, but be careful, because if you were relying on one of his standard abilities to help you pick up the kill, the silence due to overheating can end up denying you a kill.
Consider leveling Scrap Shield for a low cooldown movement speed boost, and an effective clutch shield. You can also use it to always stay above 50% heat.
The Equalizer receives no benefit from being cast above 50% heat, so it can be wise to cast early so that you don't risk being overheated when you really need The Equalizer.
Proper placement of The Equalizer can completely change a teamfight. Depending on where it is placed, Rumble can punish enemies attempting to enter a teamfight, damage most enemies already within the teamfight or cut off the escape route of enemies wishing to exit the teamfight.
When casting The Equalizer, note that the point from which the cursor is dragged will become one end of the trail.
The Equalizer has a very large casting radius. Rumble can often surprise enemies by casting it from behind a wall.