Toning up belly after baby?

Hi all, i need some tips how to tone up the belly after having a baby, well two kids actually my 1st child is 3 and my 2nd child is 13 weeks, am getting married next yr and want 2 look fab! Ive lost a stone in 13 weeks so must be doing somethin right! At the mo im doing sit ups and swimming once a week also loadsa other exercises aswell, dont no if the sit ups actually work? Some tips would be great, Nice tips and comments only please thanks