I have not done anything special to configure suspend in Slackware. It's a fresh install with XFCE and I believe that the XFCE menu uses pm-utils to suspend, hibernate, etc.
I added the line to lilo.conf which did not make a difference. I don't currently have a a mkinitrd.conf (only /etc/mkinitrd.conf.sample). Should I just copy the mkinitrd.conf.sample to mkinitrd.conf, and, if so, which lines should I uncomment?
This is the advice from the arch wiki that I used to get suspend to work:
Reboot instead of resume from suspend
This problem started when saving NVS area during suspend was introduced (in 2.6.35-rc4) (mailing list post). However, it is known that this mechanism does not work on all machines, so the kernel developers allow the user to disable it with the help of the acpi_sleep=nonvs kernel command line option. This option could be pass to the kernel through GRUB options by editing the file /boot/grub/menu.lst (GRUB 0.97) on the kernel line.