There are lot's of ways to get the job done but they all rely on the same principle.
Ohms Law says that Current = Voltage / Resistance.
Power = Current * Voltage which in this case is pretty much all getting dissipated as heat in the glow plug. But current and voltage are joined at the hip because of Ohms law.
You can do a little algebra and come up with Power = Current^2 * Resistance = Voltage^2 / Resistance
So, that means you can manage voltage or current and still get power to the plug. Some options are:
*You can use a voltage divider to get the voltage and current in a range that won't burn out the plug. That's simple but it does work.
*An analog voltage regulator keeps the voltage part constant. The resistance of the plug changes a bit with its temp but as long as the voltage is high enough, you'll still create enough current and heat to light the plug.
*An analog current regulator will work too. It will automatically adjust source voltage to compensate for any changes in resistance of the load (plug). This allows you to keep the current constant as the plugs resistance increases with temp. That will keep the plug hotter than a voltage regulated source. This is the way that seems to be the best/simplest if you're going to build it yourself. The Ghost posted a schematic for a constant current source that is about the simplest you can make.
*You can also use digital duty cycle regulation techniques like pulse width modulation or pulse frequency modulation like Rodney pointed out. The idea is to average higher current or voltage values over time to get the levels you need. This doesn't work on all types of loads but it will work on a glow plug. I honestly don't know how many panels use this approach but the circuitry is more complicated and expensive than a voltage or current regulator would be.
In the old days people just connected a 1.5V dry cell to the plug. I use a rechargeable glow plug igniter with a 1.2V NiCad. It doesn't regulate current and it doesn't use pulse modulation but it works great.
Mark