How do I unstick a cpu from the heatsink?

I was working on an older system I have to get it ready for future upgrading and I noticed that the CPU is stuck to the bottom of the heat sink. What is the best way to get the CPU of of the heat sink without ruining anything? I was thinking of sliding a thin putty knife in between the cpu and the heatsink to get it loose but didn't know if that would hurt anything?

For future reference, you can usually avoid the CPU sticking to the HSF if you run something like Prime95 for a few minutes to heat up the CPU, shut down the system, and then remove the HSF while its still warm.

I second the last two posts about using heat to soften the paste. THEN...

Do NOT attempt to remove heatsink, fan and CPU from the socket right away. Instead, use the socket as a gentle CPU holder. Disconnect the brackets that fasten the heatsink down tight. Grasp the heatsink and GENTLY twist it both ways a couple of times. If the paste is soft, this will break the tight bond between heatsink and CPU, making it much easier to pull them apart when you do lift off the heatsink.

If you cannot get the twist idea to work, the backup plan would be to remove all together and try the thin knife blade idea.