While I'm not exactly a good Charger player, I have some experience learning how to use them. I might also be totally wrong in what I'm about to say.
Obviously frequent target practice is important. One exercise that really helped me is going on the ramp infront of the moving targets, and just keeping your reticle on them. The Jet Squelcher is good for it, since its range will let you consistently see when your aim is actually on the target. It's great for helping you keep your aim stable, and for letting you get used to your sensitivity. Stable aim means you won't get nervous aim when enemies close in on you.
Then there's your sensitivity setting. I keep seeing people say -3 motion and 0 stick are ideal. If you want to try that, then that's what most people think are best. I prefer +5 stick and +5 motion after getting used to it. You aim faster, which means people have less time to react to your shots. You're also better equipped to make those sick snaps! Ever splat someone doing a sweet 360 jump turn? Neither have I!
Snapping your shots give enemies little to no time to react to your shots. I would say a good way to practice this is simply to hit the targets as quickly as you can. There's right peeking, but it just seems kinda slow. May as well snap aim. Of course maybe I just don't know how to properly utilize it.
There's also using which Charger fits whatever mode. If you're having trouble keeping a good score in Turf, then the Firefin can help. You can fairly consistently score over 1000 points with that thing. I wonder how well you can do with a Firefin using some Special Charge Up and Tenacity. I should try that. I think the regular Charger is the best for Tower Control. You're pretty well equipped to harass the tower. In Rainmaker, I would only use the Squiffer. You have to move alot, and the Squiffer is best for staying mobile. If you want to challenge yourself, or make your enemies underestimate you, use the Goo Tuber.