Pls don't ever let them get so long as to curl into a hook! That is very dangerous for them and can be painful. She cries because the quick is probably higher in that toenail. When you don't trim them regularly, the quick keeps growing further up the toenail. Then trimming is painful because there are live nerves there.
Here are some directions for trimming:
How to Trim a Dog's Toenails | eHow.com
The suggestion in there for using a human nail clipper is a good one. I actually have a baby nail clipper I use sometimes. It is sharper than any of my adult size clippers and makes the job easy. Plus, you can't take off much at a time with them.