As you've given no idea of your scenario, let me make some suggestions. If there is no "equipment" involved, my suggestion would be to get the ticklee lying on their stomach; then sit on their legs (calves) and use your legs to restrain their feet and ankles. In this manner your weight is all to your advantage, preventing their feet from being anything other than "straight" while you tickle. If "restraints" are available, then, while in the same position, tie their big toes together. This is a much easier situation than trying to manage a (presumably) resisting ticklee who's on their back. Further availability of "equipment" opens a range of possibilities.