Seat people man, woman, man, woman when possible. If applicable, the host and hostess or co-hosts are seated opposite one another as a rule. If there is more than one table, be sure that there is a "host" for each table.
Most people will automatically group themselves with other people of a like gender. As the host, it is your job to break through this cliché so that all of your guests can enjoy dinner more. Ensuring that people sit in a place different than they may have chosen themselves will keep your guests on their toes, engaged, and socially inspired.