Some european tips for this problem:
First I recomend a product that removes brake dust, called carpro Iron-x. Cleaning with this product removes every stain of road salt, brake dust and pretty much everything else. Let the product work for 3-5 minuts. Don't apply in direct sunlight. This also desolves any layer of wax and other protective compounds.
Second advice for easing the removal of brakedust is a good car wax.
When waxing the car you apply a protective coating over the paint. If you have a good wax and clean the often, the brake dust won't have the possibility to stick to the paint.
I reccomend collinite 845, for a cheap and easy wax to work with, but most waxes do the same job. Only difference if how long the last. anything from a few weekĘs to 5-6 months is normal.
Next advice on how to avoid brake dust in the paint is to buy a set of "dust free" brake pads.
EBC makes brake pads that makes very little brake dust. They call them green stuff. I'm sure they'll make some for the new IS soon.
The yellow\rust collored stains that blksolara is talking about can actuallly be ressidue from road salting or dust from spiked tires if they do have these things in the winter time where you live. A good degraser might actually remove these stains completly, even if it looks like rust. In northeren countries we get a lot of these stains. Very visible on bright colors. If your rear bumper have them, you'll know it is not rust stains, since the bumper is plastic.
Only way to remove these stains are with a strong degreaser that disolves them. They will look like rust until they are completly removed.
They look like stone chips that have begun to rust, but you'll feel that it is a spot on top of the paint instead of a hole in the paint.
They can not be polished away either, because they are very very hard and durable.
A good degraser is alfa omega when it comes to removing brake dust and other kinds of spots from white paint.
when it comes to sports from bird poo, crushed bugs and that kind of ressidue, I recomend a degraser named Bilthamber Surfex HD. This desolves those kind of spots very easy.
Try searching the UK detailig forums for more tips.
Here is a tread in a norwegian detailer forum about the exact same problem, use google translate to read it. They basicly recomend strong desolvants and clay bar kits.