What are the best charging practices to make my lights last?

Li-Ion cells are actually much more robust than people think. Cell phones tend to have short battery lifespans as they are devices that get used literally every day and fast charged constantly, while using Li-Po packs, which degrade more quickly. Li-Ion cells (at least the ones we use from LG and Samsung) are rated to hold 80% of their initial capacity after 1,000 full discharge/charge cycles, and that's at 1C charge rate (aka "fast charging").

To keep your lights lasting the longest for years and years, there's a few things you can do to minimize wear and keep the cells healthy.

First, keep the lights charged, just, in general. That means if you finish a ride and you've only used half the battery, charge it up. You don't need to run the battery dead first as Li-Ion cells have no "memory" like old Ni-Cd battery chemistries. Whenever you're done using the light, charge it up.

Similarly, the fastest way to kill a Li-Ion battery is to leave it dead for extended periods. This is the most common cause of runtime issues on lights, not regular usage. So if you're storing the light for a few months, then make sure it's fully charged before putting it away. If you can, check it once every few months or toss it back on a charger for a bit. You can even leave it on the charger indefinitely and it won't hurt anything, as the light controls how it charges. 

If you really want to nerd out on extending life cycles, then use a slower charger. The faster you charge Li-Ion batteries, the more they degrade, so if you were to use a 0.5A (500mA) charger instead of a 3.0A charger for those 1,000 cycles, the degradation would actually be far less than 20%. If you need fast charging in the middle of a race or for pass-thru charging, by all means do it, but if you're just charging at home overnight for the next night's ride, then a slower charger will put less stress on the batteries as well.

Other than that, just keep the lights indoors in stable temperatures when not in use, as big temperature swings to extremes, like in a garage that gets below freezing in winter and >100° in summer, will stress the cells more. Keeping the light indoors and charged up will keep them healthiest for sure.