Someone had a guide to doing that around here... Basically you need a hair dryer (one with a hot setting), some alchohol (spelling), an exacto knife, a thin piece of hard plastic or glass (a credit card will do), and a steady hand.
Use the hair dyer to heat up the heatsink on the NB, using the credit card or glass or plastic to shield the rest of the motherboard/components from the heat of the hair dryer. Slowly and carefully use the exacto knife to cut into the paste/epoxy that is between the heatsink and the chip, again using the piece of plastic to shield other components on the motherboard (you don't want to cut through a trace on the motherboard). This will take a little time, and be careful with the exacto knife. The best way is to slowly make your way around the entire outside of the heatsink with the blade, cutting into the epoxy just a little bit at a time. The heat from the hair dryer helps to soften the epoxy a bit, but you need a high heat dryer to really do any good (just remember you can damage the board with too much heat, the board and other components are not usualy subject to high heat). You will eventually be able to work the heatsink off.
Once it is off, you will need to use the alcohol to remove the rest of the epoxy (you may also need to use some sandpaper). Now depending on the NB, you might want to lap it a bit, but lapping a NB is much harder then lapping a heatsink, DO THIS AT YOUR OWN RISK.