A parry is a push or deflection as opposed to a full out block that often meets power with power. A very simple technique I learned from a Kenpo book is to parry and then add a block with the other arm. Without picture or video, let's try to go through this.

You sense/see a right fist moving towards your head. You step off the line of fire and deflect the punch with your left palm. As you do that circle your right arm into a block of the same arm.

This puts your hand into position to grasp your opponents wrist. At the very least he'll be slightly off balance and unable to attack towards you. You can follow up with a armbar strike to his elbow, a joint lock, or your own strike with a knife hand under the nose or throat.

Here is an older video I did where I demonstrated this. Obviously this is a demonstration. Daniel held the punch out there so you can see what is going on. In the real world a punch goes in and out fast. However, this works with a jabbing punch, but you have to be fast, alert, fluid and have good hand/eye coordination. We also did not rehearse this before hand, so he didn't know what I was going to do. Daniel is a United States Marine now, and is originally from Brazil.