When it absolutely needs to stay on forever, Arctic Silver's Arctic Alumina premium ceramic alumina adhesive is the way to go.  This high temp adhesive will ensure that the heatsink will stick to whatever you attach it to.

The ceramic composition means that there is no danger of a short circuit, unlike the silver based compounds. Arctic Alumina conducts heat away from sensitive components and improves the efficiency of your cooling system. Keep things cool and functioning under the hottest action!

 

Characteristics:
Arctic Alumina Adhesive uses a layered composite of aluminum oxide and boron nitride.
Arctic Alumina Adhesive is a pure electrical insulator, neither electrically conductive nor capacitive.
A set of Arctic Alumina Thermal Adhesive consists of two tubes containing a total of 5 grams of adhesive (2.5 grams each of parts A and B) and a plastic reusable mixing wand. (Approximately 3cc total adhesive.)
Although lighter per set than Arctic Silver Adhesive due to its use of ceramics rather than silver, each set of Arctic Alumina Adhesive  contains the same amount of adhesive by volume as a set of Arctic Silver adhesive and will cover the same area.