There's quite a few things you can do before the covers are fitted, ie abrasives, patinating fluids etc. However as they're now on your pickups more care needs to be taken.
What I tend to do is carefully abraid the covers with very fine wire wool-this takes the sheen off the nickel-one or two gigs later, plenty of sweat and they'll look like they've been on there for years 8) .
We're in the process of having tooling made to produce exact replicas of the original PAF cover. These will be available in raw nickel silver or nickel, chrome or gold plate. We're very excited about this as no one has really got this right yet, certainly not with any consistency. Watch this space :D