How to find hidden space wasting files?

[Josef Lowder]

.
Recently, I've noticed my system seems to be generating 
some files that are chewing up space, but I can't find 
where this is happening.  When I reboot, the wasted 
space is reclaimed/restored, and then it gradually builds 
up again. 

I've deleted files in various "tmp" and "temp" directories 
and cleared 'cache' but that does not find and reclaim all 
the space. 

Where can I look to find all the files that the system is 
generating?  And how can I modify the system so it will 
keep from wasting so much space? 

After my last previous reboot two days ago, 'df' showed this: 

Filesystem            Size  Used Avail Use% Mounted on
/dev/hda2             9.9G  2.6G  6.8G  28% /
/dev/hda1              76M  5.3M   67M   8% /boot
/dev/hda3             9.9G  7.5G  2.5G  76% /home
/dev/hda7             9.9G  7.3G  2.1G  78% /mnt/hda7

Today, before I rebooted again, it looked like this: 

Filesystem            Size  Used Avail Use% Mounted on
/dev/hda2             9.9G  2.6G  6.8G  28% /
/dev/hda1              76M  5.3M   67M   8% /boot
/dev/hda3             9.9G  8.5G  1.4G  87% /home
/dev/hda7             9.9G  7.3G  2.1G  78% /mnt/hda7

Today, after rebooting again, it looked like this:

Filesystem            Size  Used Avail Use% Mounted on
/dev/hda2             9.9G  2.6G  6.8G  28% /
/dev/hda1              76M  5.3M   67M   8% /boot
/dev/hda3             9.9G  7.5G  2.5G  76% /home
/dev/hda7             9.9G  7.3G  2.1G  78% /mnt/hda7

Notice the change in my /home directory from available 2.5G 
to 1.4G and back to 2.5G after rebooting again.