what works for me is that my trainer tells me shoot it out nice n strait and with your muscles loose and when you get close to the target tense and snap the fist..
nice n powerful, when i was tryin it first it was a lil hard to get perfect every jab but i found when shadow boxing throwing the punches right everytime and getting used to it more helped me put hard jabs together, the same applied with strait right hands, can't get enough of strait right hand leads now!
peace.