I like to think that I am for the most part pretty competent at working on my own bike and usually feel like I actually do a better job working on my own bike than most bike shop mechanics just because I am willing to spend lot of my own time to get things JUST right. That being said, I was pretty frustrated with setting up my Planet X brakes and couldn't get them really well centered despite spending quite a bit of time on them (there is some weird brake arm travel asymmetry on these when you use the fine adjustment screw on the caliper arm to extremes). Then I met some Planet X people at a bike show and they showed me a method to help centering that I haven't noticed mentioned online (or at least didn't understand).
RIck's method above is pretty spot on, but in between steps 3 and 4 you can take a 13mm spanner wrench and rotate the entire brake assembly around the main fixing bolt as I tried to show in my crappy picture below. Using this I am now able to set up the brakes so that each pad is equi-distant from the rim without braking, and then each pad contacts the rim at the same time during brake lever pull-ie properly centered. The performance of these brakes seems pretty great so far.
On a side note-the brakes have a somewhat interesting design feature that I'm not sure is intentional. No matter how poorly centered I had the brakes, they never really seem to push the rim off center under actually braking loads. Seems to me that upon contacting the rim, the arms are essentially self centering under load.