There are a few way to generate more random logs.  The first is to set the Variance of the drive output ()- to make sure the values for each drive are slightly different.    You can set the variance from the My Account -> Rates/Units page.
The second is to auto-populate multiple times to create a layering effect on the driving log.  Each time you run autopopulate - it will append your log and create additional log entries.  To start another round of autopopulating, simple visit the Basic Info link.
The third option is to create repeating events using the “New Drive” feature - and create a series of drives that will help create more randomness in your log.   Click "Repeat" from the "New Drive" window and you will be given the opportunity to setup weekly or monthly repeating events.