archiving old logs

Hi,

Currently I have a system on a website where I log successful and unsuccessful log-in attempts to a html. This is done simply by finding out if they are successful or not and then opening the appropriate html file and appending their username and the time that they attempted to login to the end of the file. I have found, rather quickly, that his can get quite long. I am stumped for how to archive these logs. Does anyone have any ideas? I was thinking of say having a log last for a week, then archiving that log and starting a new one. Anyone done a similar thing? Thanks in advance.

Dave.