declining disk space on linux server

Hi,

I have a problem with my LINUX system.My free disk space has been reducing over a period of time.When I use the command df -v, I see that the volume of amount of used space on the root partition is growing rapidly and now stands at 90%.I also use my machine as a mail server.I sometimes clear undownloaded mail but it does not help much.Is there a way I can safely clear log files or anything that might be causing this problem.I have not installed any new programs for sometime.Your contributions will be highly appreciated.