In order to make sure that your Magento is not being slowed down by a huge amount of old log records, it is recommended that you set up its log cleaning function to automatically purge old logs.

You can do this from the Magento admin area > System > Configuration > Advanced section > System > Log Cleaning. Once there, modify the value for Save Log, Days. The default value is 180 but we recommend changing it to 60 or even 30 days. Then set Enable Log Cleaning to Yes and click Save Config to save the changes.

Note that the Log Cleaning functionality depends on the Magento cron. You need to make sure you have set up the cron in order for your log cleaning to work properly.

Another solution is to use the maintenance script provided by the Magento community and available at:

http://www.magentocommerce.com/wiki/groups/227/maintenance_script

You will need to set the cron job according to the instructions in the article for the log cleaning to be completed successfully.