Debian (and Ubuntu also) cleans up automatically the files found under /tmp at system reboot. This is normally a good thing but there are cases you might want to stop that from happening. This setting is controlled by the TMPTIME parameter from /etc/default/rcS. By default this file in etch looks like this:
# Time files in /tmp are kept in days.
TMPTIME=0
# Set to yes if you want sulogin to be spawned on bootup
SULOGIN=no
# Set to no if you want to be able to login over telnet/rlogin
# before system startup is complete (as soon as inetd is started)
DELAYLOGIN=yes
# Set UTC=yes if your system clock is set to UTC (GMT), and UTC=no if not.
UTC=yes
# Set VERBOSE to "no" if you would like a more quiet bootup.
VERBOSE=yes
# Set EDITMOTD to "no" if you don't want /etc/motd to be editted automatically
EDITMOTD=yes
# Set FSCKFIX to "yes" if you want to add "-y" to the fsck at startup.
FSCKFIX=no
In case you need to change the default usage just set TMPTIME to an extremely large value, like 3650 (=10years) and this will actually prevent the system to remove the tmp files for 10 years.