I am definitely NOT gifted in the Ab department..... I can sometimes get a "two-pack" -- on my very upper portion only..... and only sometimes...
I do:
Crunches
Crunches on the decline bench
Crunches on the decline bench with a 25lb plate/with my hubby tossing a medicine ball at me
Hanging leg raises (on Roman chair)
Leg raises on the decline bench
Machine crunches
I do only crunches (lightly). My lower stomach isn´t ok yet due to c-section (7 1/2 months ago), it´s still quite without feeling. I don´t know does it make any difference but surgery was done vertically and maybe it just takes more time to heal properly.