Apex: Schedule Job Every 30 Mins

/*
  Expression	              Description
  0 0 13 * * ?	              Class runs every day at 1 PM.
  0 0 22 ? * 6L	              Class runs the last Friday of every month at 10 PM.
  0 0 10 ? * MON-FRI	      Class runs Monday through Friday at 10 AM.
  0 0 20 * * ? 2010	      Class runs every day at 8 PM during the year 2010.
*/

String onTheHour = '0 0 * * * ?';
String onTheHalfHour = '0 30 * * * ?';
System.schedule('MyJobName', onTheHour, new MySchedulableClass() );
System.schedule('MyJobName', onTheHalfHour, new MySchedulableClass() );