Place flank steak in a pan to marinate. Mix all other ingredients in a bowl and rub all over steak. Let sit at least 1 hour (best if it sits overnight.) Heat up the grill. Remove excess marinade from meat and mix with 1 more cup of barbecue sauce. Simmer this for several minutes, then keep warm. Grill meat until medium-rare and let rest a few minutes. Slice flank steak across grain of meat into thin slices. Slather with sauce, and serve immediately. Accompany with baked beans and coleslaw.