I've always used autoglym bumper care. Its like a thick green paste that you rub in. Used on it on black trim on other cars and works a treat. Also will remove polish marks from rubber seals etc.
Around £6.
Ross
Another vote for AutoGlym Bumper care. It goes a long way. Just bought my second bottle after about 2 years.
A can of Back to Black will last about 2 weeks.
Back to Black is a bit quicker to apply.
It works on everything rubber or black, and is reasonably long lasting. I have a suspicion it is made of something like silicone mixed with washing up liquid. It separates out a lot and needs a good shake before use.
I use it whenever I wax the car and it lasts until the next wax - usually a few weeks.
I have also used Tesco's own bumper care aerosol cans, and they're the same - quick, but they run out quickly too.
I also have Wonder Wheels tyre shine - which is like a white milky substance. It is OK, nothing special, free with a bottle of Wonder Wheels.