Do you have a bicycle pump? Or even a straw that you can blow down?
Whereas a good blast of air will clear most dust from a heatsink, there are times when this is not sufficient. In these cases, you need to loosen the accrued dust with a small brush (the one that you sometimes get with electric shavers is ideal) prior to blowing it away. At the same time, detach the fan from the heatsink and take the opportunity to clean the blades. Don't forget to reattach the fan afterwards as even with a perfectly clean heatsink a faulty/missing fan can cause the CPU temperature to rise as the heatsink on its own is not able to dissipate the heat generated.