class MyForm(forms.ModelForm):
    class Meta:
        model = MyModel

    def __init__(self, *args, **kwargs):
        inst = kwargs.get('instance', None)
        super(MyForm, self).__init__(*args, **kwargs)
        self.fields['myfield'].queryset = inst.related_name.all()
    # or
    self.fields['myfield'].queryset = SomeModel.objects.filter(field=inst)
    # or ...