Hello, I am about to receive my repaired P3A from DJI and would like to know what is the best way to prevent cracks? Motor reinforcement plates? I heard they make it worse... help please

Don't fly too aggressively and pray to all gods you know.

perhaps reduce the gains

You might consider backing out the motor mount screws an eighth of a turn...

To reinforce the advice of Helimage, my P3P has flown since June 2015, continues to fly regularly, and has yet to develop cracks. I'm sure numerous others continue to fly with the original P3 shell.

Lucky???  -> Probably not.....

Heat and stress are two of the primary factors that contribute to shell cracks. If you take precautions to reduce these two factors then the P3 shell will thank you and will last longer.

In the P3, the battery produces a lot of heat! So having additional batteries and not trying to fly them to exhaustion can reduce their heating effects on the airframe. Removing them immediately upon landing can also help reduce the heat radiating within the airframe.

Flying the P3 like a drone racer instead of an airborne camera system can also induce increased stress on the four arms, the plastic motor mount structures, and speed the tiny stress fractures that ultimately appear as shell cracks. If you fly smoothly, pan smoothly, apply power smoothly as if you were always filming, then you'll likely reduce the many hard brakes; the sudden flight path change; and the sudden power changes that stress the plastic.

Not to take all the fun away from flying, both the P4 series and the Mavic have changed the airframe materials and design. They are engineered for sport-mode flight and are less susceptible to airframe fatigue.  If you fly the P3 within it's design limits, the shell will last longer.

There is nothing you can do, if it is bad plastic it is going to crack. Just like the plastic the remote is made of. I had never opened or dropped my controller, I had never used the RTH button either, but one day my right stick gimbal fell partially into the case. The RTH button was loose also.
I had never flown my P3 aggressively and always caught it by hand. Yet out of the blue, one day I found severe cracks on two of my arms.