One way - the crudest - is to install a Resistance in between the origin and the end (series). Other more intelligent ways include transforming the "cycle" of the AC (120 Volts, 60 cycles) via Capacitors (this will chage the relationship between the Current (I in Amperes) and E (Voltage, in volts).
For instance: if they use a big Capacitor in series with the feed of the motor, the Current will increment instantaneously, making the E (voltage to drop considerable). This will change the relationship (the more Amperes) the Less voltage.
This will be fed to a circuit that will "trigger" (let go the charge of the Capacitor, discharging it rapidly and stopping the process in the way), generating a "blow", a hit to the motor, that will last a mere fraction of the time interval, but creating enough torque for the motor to continue strong. Then the "next" pulse, and so forth, hundreds of times a minute. second or whatever. They were originally (they modified them again and again, many times, creating new things all the time!) called "pulse" motors, then, micro motors, etc
Some may suggest to add more poles, but that would debilitate the motor output: more poles, less torque. You need more current, less voltage. This is called the Cosine Phi in this relation. When you change it, you generate torque (strength). All IS a relationship.
The latest is "brushless AC motors" used in the industry to move heavy elements very rapidly and accurately.
One way is that copping a reducer gear box with ratio 1:2 or 1:4 and up .
one way is to use dimmer speed control ,but will make change to power of the motor.
If this is s big motor then get a VFD.
http://en.wikipedia.org/wiki/Variable-fr...
Note that this kind of speed control is relative regulated.
If it is a small fan etc, put a AC capacitor in series. ~motor running capacitor.
If it is a true AC motor, the only thing you can do is reduce the frequency of the applied voltage.
Use gears or a pulley system to change the speed, probably connecting a separate axle through the gears or pulleys. Reducing the speed will increase the torque.
Variac or Silicon Controlled Switch.
Add more poles
