Do you need more holding torque, or do you need more torque at high speed?
For the first problem make sure there is sufficient current in your supply to allow the coils to draw their maximum current.
The second problem is more difficult to attack since the faster you go the less torque you get. This is typical of all motors. The other problem with steppers is resonance. If you try to run at a reonance point your torque will go to zero and stay there. You must run the motor either faster or slower.