When I painted the plastic trim on my running boards, I scrubbed it with Bleche-white (Purple Power works good too), rinsed it and dried it.
Wet sanded it with #400 paper, scrubbed it again.
Used PPG DPX-801 Universal Plastic Adhesion Promoter.
2 light coats of PPG Primer, light sanding with #400 after it dried
2 light coats of DBU base coat
2 coats of clear
Boards have been on the truck since July, and no chipping or peeling, even on the inside of the mud guards, so I guess it worked.
Flex agent is not necessary on hard plastic with the PPG paint - and many body shops and some PPG techs say it is not necessary on the new urethane bumpers. The 2 keys are making sure the plastic is clean and well prepped before you start painting, and use thin coats to minimize the possiblity of chipping & peeling.