How do you get a 6 year old to stop whining and throwing fits?

We`ve tried everything. Rewarding good behavior, time outs, loss of privilages... nothing works. Its over stupid little things like having to leaving grandpa and grandmas, not getting an extra cookie, having to come inside at night, bedtime. Her 4 year old sister doesn`t whine or throw fits near as much as my 6 yr old. I`m at my whits end with her. Help!!