A nice new, hidden, feature in Microsoft Dynamics CRM 2011 is the option to automatically delete the logs of successfully completed workflow jobs. This option is found at the bottom of the Administration tab of the workflow designer form.
The System Jobs log fills up with a lot of stuff, including workflow logs. This is an easy way to reduce it. However, be aware that you will no longer be able to see a record of a workflow having completed, neither in the System Jobs log, nor in Workflows on the affected records. So you might want to be sure a workflow is performing properly before setting this option. Also, the option is set on the individual workflows. I haven’t seen a global option to do this for all workflows.