Technique will largely dictate your power, there are many types of jabs and thus many techniques: snapping the punch, underhand, vertical fist to split guards, only partially turning it over to reduce telegraphing with the elbow, and more. The technique Ive found to really increase how stiff my jab is is this: Turning the shoulder of course, having it set and slightly leaning forward.

-Setting the shoulder forward like the top bit of a pushup, using your serratus.
-The lean forward is like you where doing a db side bend is probably the best way to discribe it in text.

So turn the shoulder, set it (both simultaneous) and near the end youll be leaning forward (side bend) a bit and connect. Connect too soon and you lose alot of power but get the timing of everything right youll feel the hit all the way down your body because there is zero give.

Other than technique start spending in the neighborhood of 3 rounds just working your jab on the heavybag after your regular rounds. I will switch from doing singles to doubles for 30 second intervals and occasionally triples. Remember to circle to both sides and always strive for perfect form. I prefer using the heaviest bags in our gym (150lbs I think....) for this.

And theres Ross's isometric punch + med ball punches. That plus speed drills.