If you dont have a pressure cooker (PC), or cant fit your substrate jars inside your PC (thats me), you can use a sterilization method called tyndallization. Its a little faffy/time consuming but really keeps those contams at bay.
Tyndallization involves steaming your jars for 20mins, three times, on three consecutive days. In-between steaming, let the jars rest at room temp.
Basically some bacterial spores can survive high temperatures, but the heat shock will make them germinate - now they're vulnerable to the next steaming session.
Even late germinating spores will be killed-off during the third steam, leaving your substrate contam free : )
I like to steam for 45mins on the third occation just to be sure!