To increase the acceleration of the dog sled, the best option would be to remove a dog from the sled.
Acceleration is influenced by the mass of the sled and the force applied. According to Newton's second law (F = ma), if you decrease the mass (e.g., by removing a dog), for a given amount of force generated by the remaining dogs, the acceleration will increase.
Adding more mass to the sled or running it up a hill would decrease acceleration, while adding another dog would increase mass and also decrease acceleration.