What is a method of insulation that can completely eliminate heat loss by conduction?

The vacuum or Dewar flask is a hollow, double-jacketed glass tube, whose interior has been pumped out to very high vacuum. Conduction can occur thru the lip of the flask (Dewar flasks can have frosty mouths) but conduction cannot occur across the vacuum of the glass walls. Sometimes, your Dewar $\text{goes soft}$, i.e. the vacuum is lost, and its insulating properties are lost.