autoglym bumper & trim gel is good for that, and makes the black bits look much better.
http://www.ebay.co.uk/itm/Autoglym-Vehicle-Exterior-Valeting-Plastic-Rubber-Bumper-Restore-Care-NEW-/261365991350?pt=UK_Car_Accessories_Car_Care_Cleaning&hash=item3cdaa087b6