Unexpected space consumption-Collection of common programming errors

I have a problem. I run out of disk space all the time. I don’t know exactly what files consume all the space. It seems that there are bunch of little files, so it’s hard to find with du command. Every time I find something to get rid of to gain some free space (mails, logs, old archives and rpms), disk is full again after some time. Space is stolen from system partition (/). Results of df command:

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1              4061540   3848404      3492 100% /
/dev/sda3              4061572   2441348   1410576  64% /home
tmpfs                   517636         0    517636   0% /dev/shm
/dev/sdb1             10317828   1894904   7898808  20% /var/lib/mysql

What would you recommend in this situation?

It would be great if you can also enumerate a list of cases when you encounter excessive or unexpected space consumption. It might help to resolve problem

OS: CentOS 5.