Good ways of trying to prevent cracking the T-bird:
1. Make sure you know how every gizmo goes before you start mounting the hsf.
2. Use the phase-change tape that is pre-applied to most heatsinks, unless you have some high quality grease (like Arctic Silver) to use instead. Silicon grease is a bit too liquid for the hot AMD core.
3. Make sure the heatsink is flat on the CPU before trying to mount the second clip. Also, avoid mounting it the wrong way around.
4. If you feel you have to apply to much force, you probably do. Abort, remove the hsf and pre-bend the clips (I always do that myself before even trying to mount it).
5. Use only as much force as necessary (that is, keep an eye on the clip so that you don't push it sideways instead of where it's supposed to go).
Note: most people seem to crack their CPU cores when removing the heatsinks.