Lyuu and Wu proposed a new insider attack to forge multi-proxy multi-signature in Hwang-Chen's scheme without clerks. On Hwang-Chen's scheme with clerks, they also proposed their improvement. Except the extra computation cost announced by Lyuu and Wu, the communication cost cannot be reduced in their scheme while the cost can be reduced in Hwang-Chen's scheme. Moreover, their improvement needs a center which is more trustworthy than the one in Hwang-Chen's scheme. To remove extra costs and overhead of trustworthy centers, two efficient improvements are proposed.