Actually Slackware should have these to clean up /tmp already at bootup time in your rc.M file in /etc/rc.d:
# Remove stale locks and junk files (must be done after mount -a!)
/bin/rm -f /var/lock/* /var/spool/uucp/LCK..* /tmp/.X*lock /tmp/core /core 2> /dev/null
# Remove stale hunt sockets so the game can start.
if [ -r /tmp/hunt -o -r /tmp/hunt.stats ]; then
echo "Removing your stale hunt sockets from /tmp."
/bin/rm -f /tmp/hunt*
As far as your /var directory, most of the time you can clear out your log files. Usually Slackware has logrotate enabled to do this for you. I personally made my own scripts to clear them up and back the files up. I'll tend to review some of my logs and if see nothing happening out of the ordinary, I'll run my script to clean the logs, restart syslog and be on my way, then usually deleting my backups after they've gotten older and I've added several other backed up log files.
I'd say if your not too worried about log files if you shutdown your machine when not using it, you can easily clear out the logs and tmp at shutdown, just removing them totally off your machine.
I can give you snippet of my script you could use to clear out the logs if you wanted?