What do I mean by this? I'm talking about how aggressively you attack your mouse button when you fire. With real-life firearms, you are taught to “squeeze” the trigger gently or gradually rather than “pulling” it as hard as you can. Why is this? By squeezing the trigger, the firearm is not going to move around nearly as much as if you were to pull it quite suddenly. The same thing applies to your mouse. By lightly tapping your mouse button, your less likely to throw off your aim than if you were to jam on it with all your strength.
Now I know this can be something hard to overcome in the heat of the battle, but by lightly tapping on the mouse button, you'll find that you will fire rockets more when you intend to and not when it finishes reloading as it would if you were to hold down the mouse button. The same applies to nearly every other weapon except the Minigun (duh) and the Link Gun. If you can't master the light tapping, you might want to try binding your fire button to a keyboard key, so that firing doesn't move the mouse at all. Try it! You'll be amazed with the results.