The particular printer I got have a driver motor for the printhead that is 24V. The high voltage motor a nice property, as it means it can be used as a generator with reasonable output voltage.
The first step is to strip down the printer to its bare parts:
Remove just about everything until the printhead is ready accessible. I removed all paper tray and associate electronic and power supply. All you need is the printhead, the rail the printhead sits on and the motor that is attached to it.
The idea is to move the printhead back and forth by hand to generate electricity.
Now you need something to store the generated energy in, so you won't have to crank the printhead continuously. I chose super capacitor as they have almost infinite life. You can use battery here too, if you don't have access to super capacitors.
I used two caps, the first one is a low impedance unit rated at 0.47F at 5V. This particular capacitor has impedance less than an ohm.
The second cap is the bulk storage cap. This one is rated at 1.5F at 5V. The impedance is around 100 ohms.
The combination of a low impedance cap with a high capacity cap is to ensure good power storage density with a low impedance path to absorb that sudden jerky movement that put out a lot of current for a short duration.
Since the output of the motor will be going from a positive voltage to negative voltage as you move the printhead back and fourth, there need a way to ensure the capacitor always gets charged, instead of being discharged when the movement is backward. A small bridge rectifier works well here. (If you don't have access to one, you can always make one from 4 diodes. Try google it)
Last is a radio. Any battery operated radio will work fine. I got a unit that runs from 2.2V to 6V. This is a nice property as it allows the capacitor to charge to a much higher voltage and slowly discharge over time, giving it long running life.
Now, putting everything together:
My little unscientific test results in a run time of about 10 minutes when I crank the capacitor to 5V and let it run down to 2.2V.
I added a small LED and resistor (4.7Kohm works well here) to ack as a cheap charge indicator.