Which item help maintain even contact and heat distribution between the CPU and the heat sink?

A good heat sink compound, often known as thermal grease, will provide an inexpensived, readily available, and reliable solution to both the need to maintain even, overall contact between the CPU device's heat-radiating surface and the heat sink itself as well ensuring proper transfer of the heat generated by the device's operation. Typical heat sink compounds are formulated from silicone and are the type most-often recommended by manufacturers and maintenance organizations (primarily due to superior performance and operational life), but there are a variety of types available, produced from petroleum derivatives as well as other substances. Any of the types available for public use tend to be a bit "messy" to work with, at least for the novice; it is for this reason that handy "syringe-style" applicators are popular sellers. A thin coating should be applied to the CPU device's heat-radiating surface (some authorities maintain that it is best to apply thermal grease BOTH there as well as to the heat-sink contact area); any excessive amount that becomes extruded out beyond the contact areas of device and heat sink once the two are firmly attached by the latching device employed in a given configuration will fail to enhance the desired effect of heat transfer, and upon occasion can actually cause problems due a tendency to decompose---and even lose the property of electrical inertness inherent to these products---under such variables as ambient temperature & other environmental conditions, and the user should exercise caution in both the amount of compound used and the areas with which it is allowed to contact. The user should be aware that the properties of thermal greases will degrade over time and require re-application of a new layer; old, broken-down heat sink compound should always be thoroughly removed first by cleaning both surfaces with alcohol or an adequate solvent (carefully-applied acetone---often available amongst everyday household products in the form of fingernail-polish remover---is a favorite product of choice for many experienced technicians