I've yet to find a permanent solution to be perfectly honest with you. The best stuff I've used so far is the Megs Gold Class Trim Detailer, but that washes off eventually. Mind you my bumpers are really faded, but you'd never tell.
The trick is to apply it with a sponge, not a cloth, and it'll get into all the deepest parts of your bumpers. If you're starting with a new sponge it'll take a while to soak up enough of the stuff to do a decent job, but once it's saturated it just wipes on. And of course it helps if you scrub the bumpers to get the dirt out from between the textured bits, otherwise you're applying it to filth not plastic and it'll wash off quicker.
I've spent a decade reviving my bumpers and I've yet to find anything that lasts longer than a month.