Hello Friend
You have to set the slave switch's priority to be lower than the master switch.
3750-Stk>show switch
Current
Switch# Role Mac Address Priority State
--------------------------------------------------------
1 Slave 0016.4748.dc80 1 Ready
*2 Master 0016.9d59.db00 5 Ready
This following link is a good read
http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_example09186a00807811ad.shtml#pval
Thanks
Suresh