First, you have to determine how the awards will be made.  Just because you're having a car show doesn't mean you have to give out trophies, dash plaques, and door prizes.  But they are a good idea, and not very expensive to do.  As a matter of fact, if you know how to do it, it doesn't have to cost you a thing.  Sponsors can pick up the tab on most of the awards.  Ask different automotive parts shops, restaurants, car dealers, just about anybody if they would like to sponsor a trophy.  All you have to do is have the trophy made and have the sponsor's name engraved on the brass plate telling what the award was for.  Make sure the sponsors get announced when the award is presented.  It's a lot of advertising for the price of a trophy.  It's also a good time to ask if they'd like to give a door prize, if you plan on giving out door prizes. 

You may also consider giving out dash plaques to a certain number of registrants, like the first 25, 50, etc. depending on how big you hope the event gets.

Judged Event

By far, judged events can be the best way to give out awards.  Naturally, judges have a tendancy to give higher marks to people they know.  Unfortunately, with smaller, local shows, the chances increase that a judge is going to know some of the participants.  If you plan on having a judged event, make sure you have judges that know a little about how to judge cars.  There's not a lot of information available on car show judging and there are a lot of variables to consider, especially on the level of judging being done.  You don't want to have a local car show judged like a concourse event.  There are several websites that have information on judging cars and scoresheets you can use.  If you are going to have more than one judge, make sure all of them have similar backgrounds in judging events and agree on how thorough of a judging each car will receive prior to the event.  You might want them to read my Advice for Judges section. 

Make sure the judges know what is expected and make sure they know how to judge.  If you are having separate awards for cars and motorcycles, make sure the judges don't give an award to a motorcycle that is supposed to be for a car.  Likewise, make sure they don't give a Best Ford Award to a Camaro or a Best Mopar award to a Lamborghini.  I've seen both and it really destroys the credibility of the judges when this happens.

Also, make sure the judges are controlled in some way, especially when a sponsor is a judge for an award they are giving out.  On the first page of the Advice to Participants section, I mention a red Mitsubishi Lancer that I felt should have gotten an award for best graphics.  That award was presented by and judged by a graphics shop.  Guess where the winner had their graphics done.  Guess where the red Lancer didn't.  The sad part about this saga is that everyone knew it.  I haven't seen that red Lancer or his group of friends at a local show since. 

The moral of this story is, promoters, hosts, and sponsors need to make sure the judges they choose know the ramifications of their actions.  Judge fairly and the show may become a huge success.  Judge poorly showing favoritism and the future of that show will be in question.  If the judging is very poor, the promoters, hosts, and sponsors may know the status of next years show before this one is even finished.

Participant Voting

Personally, I've never been a big fan of participant voting.  If the participants don't constantly walk around to see all of the vehicles in the show, someone who gets to the show early stands a much better chance of getting votes than someone who gets there later in the registration time frame.  Participants like to go around after they get there, vote on the vehicles already there, and turn in their vote card.  If a car shows up that they may have liked better than the one they already voted for, it's too late.  If a large club shows up, you can rest assured they will have it planned for whom they are going to vote.  Some clubs actually plan on who is going to get a trophy at which event based on their voting alone.   If you plan on the participants voting, make sure you have someone to count the votes, especially on larger shows.

One method of using participant voting that is becoming more popular is to not allow someone to vote for any other vehicles in their class.  This keeps down some of the favoritism voting provided the classes are broken down appropriately.  As an example, if you are entering a Corvette and there is a Corvette class, you cannot vote for any vehicle in that class.  If you have a Muscle Car, you can't vote for any of the other cars in the Muscle Car class.

Spectator Voting

Spectator voting is usually a pretty good method of giving out awards, especially with larger shows.  Just be aware that some participants, especially those that reside close to the location where the show is being held, may have lots of friends and relatives show up specifically to vote for their car.  Larger shows that attract spectators from a wider area tend to compensate for local favoritism.

Combination Voting

Because participant and spectator voting can usually be easily manipulated, some shows use both in determining the awards.  Most shows that use this method count up the votes separately and either give separate awards, ie 1st, 2nd, 3rd Place by Spectators or Participants, or they add in the participant votes to the spectator.