Custom Schedule Classes

If you ever have to shceduled a class but provide a specific custom shcedule plan run a bit of code in the System Log, similar to

string CRON_EXP = '0 30 * ? * MON-FRI';

string jobid=System.Schedule('<<any name>>',CRON_EXP, new <<name of class>>());

This will run the class every 30 mins passed the hour from Mon-Fri

Changing the 1st line with

CRON_EXP = '0 0 * ? * MON-FRI';

will run the class on the hour 

More info for specific customisation at     http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_methods_system_system.htm