once you have set the heatsink, try not to move it around and if you absolutely have to apply pressure sparingly to the top of it evenly, not on one corner or side as this will increase the chances of a cracked core.
dont be afraid to muscle on the heatsink clip (if it has a clip), it can be tough.
if you use a screwdriver or something, make sure are not in the path of a cap, you dont wanna end up taking one out
thin layer of thermal grease, not a big blob approach (unless its a thermal pad, then it doesnt really matter )
it may be easier to install before putting the mobo in the case