Take the Wild Cohort feat and retrain its feats into Martial Study and Martial Stance to grab Island of Blades. That way, you count as flanking any time you both threaten a foe, which will basically be all the time.
Also, a ring of the darkhidden for 2000 gp from the MIC, and a wand of that cantrip that causes absolute darkness? I think it's called No Light. Just make sure you have darkvision. It's better than a ring of invisibility for this purpose, since it's much cheaper and doesn't crap out after you attack. Not so good at actual invisibility, though.