I do not recommend it, but you can put

rm -rf /tmp/*

to your

/etc/rc.d/rc.local

Yeah its not recomanded and maybe you should use tmpwatch to do it (http://linux.about.com/library/cmd/blcmdl8_tmpwatch.htm) and delete only oldies :-)

But first question is : why do you want to remove all the temporary files ? Just to save space (Did you tried to look at cache files too ) ?

Ex : yum store his cache at /var/cache/yum and this cache can be sometime huge ...

Instead of deleting temp files, if you have enough RAM you can put the whole directory in the memory. It's much faster and there is no need to delete anything as they go away any time you reboot ;)

Just add the following line in your /etc/fstab:

tmpfs /tmp tmpfs defaults,noatime,mode=1777,size=1000M 0 0