First of all, it doesn't charge "better", when You discharge it to ~318-something mV (cause phone shuts own then). Its plain bul... wrong assumption. Just remember, than green led *doesn't* mean that charging is finished. It may be as good still charging, already charged, or infinite charging via too much system load - I don't want to get into too much details here (if You even gonna be interested, You'll know where to ask).

Second thing - Li-Ions (in this case, Li-Po) doesn't like to be completely discharged. to be honest, they hate it and tend to die after just a few such incidents Thankfully, both pcb in battery and N900 internals doesn't allow You to discharge battery completely. Most devices assume that @ 3.0V, Li-Po is discharged "to 0%" and shouldn't be drained more, to avoid possible damage. In fact, absolute safe minimum is 2,5V, but from that state, battery may drain itself (on idle) to non-safe voltage very quickly. And, in our stock Nokia battery, from 3,0 V to 2,5 there is only 5 mAh of power, so no much gain in sucking it, either.

Discharging battery (almost) to relative 0% may be useful in some cases - i.e. charging chip calibration - but it's also totally different topic.

Anyway, Your question is rather strange, even if We don't mention all this geeky stuff. You want to discharge Your battery completely (relative)? Just leave phone will full brightness, screen on, and video playing until it turns off

If, by any case, You would like to get possible filesystem corruption and to put Your battery in danger, You can also execute "stop bme" as root, then just use Your device until it "die" from low power (@ about 2850 mV). Not very much sense in doing so, but if You want anyway...

// Edit

To finish this post with something useful - Li-Ions and derivatives just love when you keep them between 30-80% (i.e, the live much longer if You never discharge them "completely", and never charge to absolutely full). So, except calibrating chip, it's very healthy (well, at least for battery) if You charge it even not fully discharged. Remember, it's *not* ni-cd...