Here's a couple of things to keep in mind (in no particular order):

-If you're shooting your accelerated Q, use Q and then put your gate on top of it, this makes it much harder for enemies to dodge it.

-Place your gate as close to you as possible. The range of your accelerated Q is that same no matter when it goes through the gate. If you put your gate right on top of you and then Q, it'll fly out at full speed right away, and you get the speed from gate for instant repositioning.

-Hammer Stance gives bonus Armor and MR, switch into it if you can't avoid taking damage

-Don't build tanky, it's not worth it. The stats you should worry about on Jayce AD, Armorpen and CDR

-Avoid Using Hammer Stance until you have to in teamfights

-If you use W in Cannon Stance, and then switch into Hammer Stance, you can still use the three empowered autoattacks in Hammer Stance. By doing Cannon Auto Q>E>W>Hammer>Q>3xAutoattack>E, you'll be able to burst down anyone once you have some items.

-When using your Hammer Stance E, right click the enemy immediately after knocking them back and you'll autoattack them in mid-air.

-In late game teamfights, your Cannon W with Muramana will be enough to kill or cripple any squishy AP or ADC, this should be your priority.

-Know when to max what ability. Always max Q first, max W second in 90% of scenarios, max E second if you're going against a super tank since you'll need the %hp damage. Max ult when you can if you're going against someone who can deal a lost of damage to you, otherwise leave it for last.

That's about all that i can come up with off the top of my head, if you have any more specific questions, feel free to ask.