is there any way to keep plastics, like the bumper covers to keep from doing the werid wavy wrinkles?

or is there any way to flatten them out after it happens?

 

Wrinkles in plastic parts are due to the residual stresses induced in the manufacture and accidental contact. Given time and temperature residual stresses always win.

 

Basically, weathering and sun did that. Sometimes you just cant help it.

 

theres the other part of fixing it too

 

Plastics tend to be very long chain polymers (n=30000 to 100000). The molecules themselves are not straight, so when you form them into complex parts with curves and angles you necessarily have to bend these same molecules, thus inducing residual stresses. You can never remove all of the residual stresses and thus there is no possibility of removing the wrinkles once they have formed.
Aftermarket parts of carbon-graphite composites would be your only hope. Think of cost of the order of magnitude of Shuttle leading edge tiles.