Below is a simple procedure to move the /var to /usr/var and /tmp to /usr/var/tmp. This is best to do early on in a new system installation since many services tend to hook into /tmp and/or /var, and may thus lock files in those directories making the move more difficult. If you’re making this move on an established system, at least stop all the services that might interfere with the process (such as database services). It might even be a good idea to boot into a single user mode (if you do so, remember to correctly mount your disks before proceeding). I usually do this early in a new system install, before installing any major services, or at least before scripting them to run.

1. Move /var to /usr/var
   
mkdir /usr/var
cd /var
tar cvf - . | (cd /usr/var; tar xvf - )
cd /
chflags -R noschg /var
rm -rf /var
ln -s /usr/var /var

2. Move /tmp to /usr/var/tmp
   
mkdir /usr/var/tmp
cd /tmp
tar cvf - . | (cd /usr/var/tmp; tar xvf - )
cd /
chflags -R noschg /tmp
rm -rf /tmp
ln -s /usr/var/tmp /tmp
chmod -h 777 /tmp
chmod 1777 /usr/var/tmp