Best way I know to lose stubborn belly fat is to walk/run/swim/bike hard for 30-45 minutes daily, 5 or 6 days a week. Plus lift weights for an hour or so 3 days a week. This will fire up the furnace that will burn that fat off..spot exercise can't really do it.

On the other hand, if you want to build really strong abs about 20 to 30 minutes daily of ab-focused pilates work on an exercise ball plus some 'Janda' sit ups will fix you up. Janda situps are killers, but they really work.

Janda Sit Up