...as a rule of thumb for ANY rechargeable device you should always charge it fully and drain it down until it dies at least once a month. this will cycle the battery and give you longer lasting batteries.
Don't follow this advice, it is wrong wrong wrong. In this age of ultra-portable, battery powered devices, you owe it to yourself to get educated, so don't take my word for it, and especially don't listen to advice like the above. Do some research yourself and get the facts. Having said that, I'll state the facts below, because I think most people won't bother to dig further! :-)
If you want to look after your battery powered device, first you need to find out what sort of batteries it uses. Most modern electronic devices use Lithium-ion batteries, so I'm going to talk about them. Note, this advice applies only to Lithium-ion batteries. These are the sorts of batteries you will find in your cell phone, laptop, iPod or Tesla Roadster.
1. Your battery has a fixed shelf life.
This has a few implications. Firstly, nothing you can do will extend the battery beyond it's shelf life. It will die, even if it is never used. This leads me on to the second point... don't buy a spare battery. When you need it, it will probably be dead, beyond resurrection. Generally, a well-cared-for battery will last five years, losing about 20% capacity per year.
2. Your battery will continue to discharge slowly, even if it is turned off.
Why? It's related to the chemical reactions happening inside the battery. However, the most likely reason is that there is special circuit in your device which monitors the battery voltage. Why do you need such a circuit? Read on.
3. Your battery will be damaged if it is discharged below a certain threshold, or overcharged.
For this reason, most devices have special circuits which measure the voltage of the battery, and turn the device off if the voltage drops too low. As already discussed, if you completely discharge the battery, it will continue to discharge even when it is turned off. This means the battery may in fact discharge to a dangerously low level, where it will become irreparably damaged. It will drastically reduce the life of your battery. Fortunately, the same circuit protects against overcharging, so you will probably never have to worry about that.
The moral of the story. Don't deliberately try to completely discharge a battery. Don't let your device sit around with a flat battery for days on end. Don't buy a spare until you need it.