UPDATE. The first two charges on the extended battery were disappointing but now the third charge gave me upwards of 100 hours! Again, light usage but still impressive. I went from 2 days of usage to 4 which was a very pleasant surprise. Do these batteries get conditioned as they get used?
Yes. Battery University has a great tutorial on getting the most out of your battery, and several good tips from them and others bubbled to the surface:
1. When you first get your battery, be sure you charge it up to 100% in ONE GO, and leave it hooked up for a while after your device claims it's charged. Many people recommend 12 hours. I think that's excessive, but hey - it can't hurt. When you get a new phone or a new battery, charge it at least overnight and resist the temptation to unplug it to play with the phone.
2. For your first couple of discharges, try to run it down to the point where the phone shuts itself off (but do not do multiple restart cycles to try and drain it further, if you deep-discharge it you could ruin it). Then charge it back up to 100% (overnight if possible so it gets plenty of quality time on the charger).
3. After that, try to keep the charge between about 20% and 80% the majority of the time. Charging much above 80% repeatedly isn't good for the overall longevity battery, nor is discharging it much below 20%. When you charge it above 80%, use a slower charger if you can - not the included 1A fast charger.
4. Every couple of months, do a full-charge, full-discharge, full-charge cycle. This helps keep the phone clearly aware of the battery's remaining capacity (calibration).
5. If you're going on a trip, you can "boost-charge" the battery by charging it to 100%, unplugging it, turning it off, and immediately plugging it in again. When the light goes from amber to green, unplug it and plug it back in (leaving the phone off). You can repeat the unplug-plug cycle several times but the benefit dwindles each time.
This can increase the capacity of that single charge, but also does some slight damage to the battery which, over time, decreases its overall capacity. It may be worth it if you'll be somewhere you know you can't recharge it for a while. But if you find yourself doing this a lot, get a bigger (or extra) battery.