I used to box, couple quick pointers:

-Bring your hand back to block immediately after you throw a punch
-Instead of trying to "push" with a punch, "snap" your fist - as if you wanted to sting your opponent. (ie. knuckles verticle until the last second where you turn and "snap" your wrist - making them horizontal as they sting his face.."
-Weight always evenly distributed on the balls of your feet
-Chin tucked in, looking out of the top of your eyes
-If you're not southpaw, step into a jab left foot forward, ending the motion at the same time you snap your wrist (added power)
-When throwing a right, rotate your hips so that your back foot spins inward and is almost directly facing your opponent by the time the punch lands..

thats about all i got to say right now..