How do i fatten up my dog? please help!?

Okay so my dog shes about halfway t my knee, shes like a little poodle. She gave birth to 4 healthy puppies exactly 5 weeks ago (OCT32008) so they ahve been taking allot out of here. She is very thin i really need ehr to gain weight. We feed her twice a day. Half a big can of Purina dog food. Now that is for big dogs, so we give her half each time. so a whole can a day. most of the time she geted fed 3 times a day. i have been giving her small denta stix. one a day. Please help, shes fairly thin. youy can feel ehr ribs very well. im worried about her.