Safely using a 10440 in my LM21 (or other)

So, I ordered a LumaPower LM21 and some *fire 10440 batteries along with some NiMH. After waiting a few weeks the LM21 arrived with some issues (tail clicky doesn't work for one) and the reseller has sent out an RMA.

Anyway, this will be my first use of unprotected Li-Ion batteries in a flashlight. What is the best and/or easiest way to keep from over-discharging these batteries when used in a light?

Assuming that the flashlight has no low voltage protection in the form of warning or shut-off, you have to monitor the usage.

I find the easiest way is to take a fully-charged battery and run it for some reasonable estimated time that you know will not totally deplete the battery, and then take the battery out and measure the voltage. Then you'll know that if you use it for some certain length of time on that brightness, the battery will be depleted that certain amount.
After doing this a number of times, and different settings(if the light has different brightness settings), you will begin to get a "feel" for how long you can use the battery safely, and not kill it. It comes with using the light.
You might accidentally kill the battery if you don't pay attention at some point, but they aren't very expensive, and if you got a reasonable number of chargings out of it, you did okay.

If you use it for EDC, you could just as simply re-charge it each day, if you used it at all. Shallow charging and discharging extends the useful life of the battery. Deep discharging reduces the useful life.
So, the general rule is re-charge it often, and get accustomed to how far it discharges the battery with your typical uses.
The volt meter will be your friend.

I don't think the light will have a safe low voltage cut since it is also designed for alkaline and NiMH. The spec shows input voltage of 0.9 - 4.2V. This would be an EDC and I purchased several of the batteries so swapping them out and charging often seems like the best way to approach it...although I do find myself from time to time in that situation where you end up needing to use your light for much longer than normal. In those cases, as you said, even if you ruined a battery you'd still be ok if you had several cycles out of it.

Side note, I wish there were some (more? are there any) lights designed for the 10440 with a reasonable voltage cutoff and/or more protected 10440 batteries. I understand the flexibility in being able to fall back to alkaline or NiMH but at least provide the light in both "flavors" and let me decide which I'd prefer.

Also, I know this thread isn't strictly a review of the LM21 and I've only had mine for a bit but I think it clearly falls into the "you get what you pay for" category. Hopefully, the tail switch is better once I get a properly working one but is seems like the length of the push is excessive.

Assuming they fit your LM21, (they're longer, of course) you might consider trying some protected Trustfire 10440 "600 mAh" - (nowhere near that) batteries from DX (sku -30788). I've been using a set for almost two years now with good results.

EZO, I may try that now that I have the light. I was reluctant to order the Trustfire with the light since the reseller had an disclaimer about them being longer and not fitting some applications. In addition they wouldn't openly say that battery would fit. But now that I have it in hand I'm 99% sure they will work.

At two for 5 bucks its worth a gamble. Be careful though, I "slightly" crushed one of them in one of my lights, although it still works fine and the PCB still functions.