Breaking off from the thread in Brainstorming about how we preconceive drivers based on their cars, I wonder if we prejudge people by how they alter their car's appearance. These have to apply whatever car they are applied to.
Some obvious ones to get the ball rolling:
OTT GRP bodykit: Boy racer. 60 on the motorway, 60 down the high street except if teenage girls in sight. Stereo bizarrely usually playing really girly R'n'B (presumably to attract said girls). Steer clear!
Loads of stickers bearing the logo of tuning companies etc: See above.
Christian fish: 50+, very slow, not very skillful, but utterly unaggressive.
(the only sticker I've ever had on a car was big picture of a devil toasting a christian fish on a big fork

on my old yellow Fiesta)
Pink furry seat/steering wheel covers, sticker saying "babe on board". Slightly aggressively driven by female under 30. Amount of pink is usually inversely proportional to the attractiveness of the driver.
Baby on Board etc. stickers. Seem to be a subconscious apologia for the driver's lack of skill. Steer clear, especially during school run!
Those yellow "25" stickers (are these everywhere or just in the westcountry?) to alert the police if anyone who looks under 25 is driving that it's probably been nicked. These mean: This car will NEVER break the speed limit even if driven off a cliff.
Two that defy pigeonholing:
Privacy glass: Can be boy racer or MPV full of kids or minor celebrity!
Private plate: Seem to be almost anyone (including me

), though illegal spacing/lettering tend to be favoured by chavs only. Often have tendency to aggressive driving (except me!)