Organizing a costume contest is a great way to liven up your Halloween party. It's really easy to do and everyone is going to get a kick out of it.
In the invitation to your party, mention that you will be throwing a costume contest. Tell your guests to dress up in the most imaginative way they can think of.
You should get some small prizes for the winners, just to get everyone to try really hard to win. You can also award gag prizes for different categories, besides the best costume, like funniest costume, worst costume, sexiest costume, basically whatever you can think of.
Make a spectacle out of giving away the prizes, do an award ceremony that everyone will enjoy.
You can judge your contest yourself, but making it a vote is way more fun. Put a ballot box somewhere in the party area and let everyone vote on who has the best costume. People will walk around, talk about their costumes, vote and have a lot of fun in the process. At the end of the night count the votes and start the award ceremony.
Remember that this is not the presidential election, it's a party, so have fun with it and enjoy yourself!