I have to say I agree with some of what ICECOLD.. says..
I am talking from ignorance however because I haven't entered a car show so
but here goes, whatever the criteria for the category and whatever the category first timers and beginners are always in danger of being put off by 'the pros' by this I mean the more experienced people with thier cars. This applies to any competiive activity, really in the interests of whatever it is you are doing ie showing your cars, minis restoration... these people need to be encouraged to participate and stick with it. So this sort of calls for a entry level category or categories.
One example of how this could be done is that each entrant initially nominates the category that they would like to enter, (this could be any of the available categories, but polite suggestions from organisers wouldn't go astray) so they enter the category, and lets say they win a prize (1st, 2nd, or 3rd) they then get a card signed to say that car and owner blah, blah, have won catergory such and such after say 3 or 4 wins they are promoted to the next category up which is a bit more competitive with slightly better cars etc. This would have a couple of benefits it would encourage people to work on thier cars to improve them so they can more up categories, each competitor would have a better chance at winning in thier category so it remains fair. Also the very best cars would be competeing against one another so it remains competitve for the top and not a case of just rocking up to collect a trophy (not saying this is what anyone does).
If a person enters a category too low they move quickly up the ranks untill they get to a more competitive category, if the person enters a category too high then they will not will much (read not at all) and should be allowed to nominate for a lower category.
This is my 2 cents but this sort of thing has worked in other sports / interest areas, but I think it would have to be well organised with in the car clubs organisation.