I helpded my brother when he entered his MK4 Golf at a show n shine recently (which he won).
Exterior:
A bit obvious, but needs to be spot/scratch/chipless and insanely shiny. No rust. Wheels both inside and out to be spotless, no brake dust. The wheel arches are easily cleaned, specially the liners! We used an old kitchen brush to scrub the paint (it's an underseal paint) so it's tough enough to shift dirt, soft enough not to remove the paint. Shocks / springs again, need to be clean, not rusty looking heaps. We painted what could be seen of the rear subframe with a black stonechip paint to cover the rusty bits. The "problem" with newer cars is they are painted underneath with a special stonechip paint which you can't shine. Older cars can be painted like the top, so can be very shiney (that's when they get the mirrors out!). Clean inside the door hinges / boot and bonnet. Paint the calipers so they look nice.
Engine bay:
Needs to be spotless. No oil or rusty bits. Best bet is engine out and paint it! Thankfully on my bros Golf, you can't see it because it's covered up.
Interior:
Spotless, no nicks or tears, no dodgy smells!
We basically spent 2 days cleaning / preparing it for the show (and it wasn't dirty before we started!) and it paid off. You have to remember, some people take this very seriously!