Some hints provided by WarnerLosh.

• Strip your kernel to the absolute minimum that will boot. Usually this is no network, and just ata/ahci and syscons (and the usual non-optional optional drivers).
• Try to suspend/resume with this. If it doesn't work, you are going to have some fun tracking it down since it may be a video card issue.
• Add drivers back one at a time until it fails. That's your driver that's bad.
• Confirm this by mixing and matching until you are sure it is this driver every time.
• Then you fix that driver's suspend/resume code.

From my experiences, problems have been 80/20 video/something else.