Power plug constantly in?

No harm. Your battery might need exercising (full discharge/recharge every week or two), but no major damage should occur. My old Inspiron 8100 has been on for over 3 years.

(You could also have your hard drive turn off after x idle minutes.)

I do the same thing with my 9200.... I have read in various PC magazines and the newspaper that it's a good idea to unplug it now and then and let the battery drain. Something about it being good for the battery. I do it every couple of weeks and let it drain down a bit and then plug it in again...

Draining/recharging

Nickel Cadmium batteries had a "memory"; it was necessary to fully drain and recharge them every 2-3 weeks in order to make sure you could continue to use them at full capacity.

Nickel Metal Hydride batteries will tend to "forget" that they can be fully charged after a long period of storage lack of exercising. You'd want to do about the same thing as NiCad batteries, drain them every 2-3 weeks to make sure that they will take a full charge. (Note this is the opposite of NiCad: NiCads will drain more quickly while operating at peak discharge rates if you don't cycle them, whereas NiMH batteries will refuse to accept a full charge to maximum voltage. Subtle, but important, distinction.)

Lithium Ion batteries require no such conditioning. As a matter of fact, fully discharging and recharging them reduces their life, since it is the number of full charge/discharge cycles which dictates how long they will last. Normally they are good for 300-500 charge/discharge cycles, or around 4 years at the outside even if unused.

Here are some good Notebook (Lithium Ion) battery care tips:
* Avoid frequent full charge/discharge cycles. Several smaller discharges and recharges are better than one full discharge and recharge.
* Avoid heat. Heat is the #1 killer of notebook batteries. You'll note that most well-designed notebooks keep the CPU and RAM far away from the battery; this is the reason.
* Ideally, you should store Lithium Ion batteries at about 40% charge. Storing them with full charge in heat destroys them quickly.
* Consider removing the battery from your laptop when on AC power. This will prevent frequent small charge/discharge cycles, avoid overcharging the unit, and avoid heat-related battery decay. Of course, most manufacturers who want you to replace the battery every 2 years instead of 3 will caution you against this with things like "moisture collecting inside the casing" or "exposing the leads to moisture". Rubbish.

That said, the gauges which monitor Lithium Ion batteries become increasingly inaccurate without full discharge/charge cycles. So in that respect, periodically discharging your notebook battery will improve the accuracy of your battery gauge, though it does nothing for the battery itself except take one more cycle away from its lifetime limit...

Redgiki pretty much covered all the most important aspects of Li-ion battery maintenance, but for those who want to know more about batteries, read this: http://www.buchmann.ca/faq.asp

There is some really useful information in that FAQ!