Reduce body fat by reducung carbs and Doing HIIT or sprints. Sit ups and crunches anren't the best way to train the abs and core. The function of the core is to hold the trunk still so that the arms and legs have a stable base to work from. The latest thinking on core traning is to train anti-flexion, anti-extention and anti-rotation movenments. These movements train the core to do what it is supposed to do.

Great articals about it here -www.t-nation.com/free_online_article/most_recent/antiab_training and here -www.t-nation.com/free_online_article/most_recent/21st_century_core_training_1

Happy training