"I keep doing the math and if you're going out for 3 weeks and need battery the whole time a solar panel may be your best bet."
Who goes out for 3 weeks at a time without some sort of food resupply? Very few. If you've got some sort of food resupply, then you might also have a battery recharge option too. I've used solar for much of one long trip, and wouldn't do so again, barring special conditions.
I've carried a smartphone on three thru-hikes now and on other backpacking trips, and Android was the OS for two of the long ones. For me, "pro-tips" isn't so much about coming up with a complete list of ways to use the phone and of which applications are best to do that — such threads have arisen before (search …) including here on BPL I'm pretty sure. What sprang to my mind was knowing the details to fine-tune things to bring unneeded battery loss down to a minimum. Beyond basics like "turn off any broadcast/receive functions, and learn how to do that in a granular way beyond just the all-or-nothing airplane-mode". I think that getting the most out of your batteries is definitely a "pro" thing, and depends in part I expect on the particular model phone you have, which flavor of Android, whether the phone is rooted or not … getting this really really right is something I've never dug deeply into, but can get pretty geeky.
Some, though, is just paying attention. For example, if you like to read at night, you can turn the screen brightness down quite a bit when it's literally dark out. Then just before you go to bed, try to remember to turn it back up, because it can be a PITA the next morning in bright sunlight trying to see what part of the screen to poke in order to get the brightness turned back up. And having a dedicated widget for that on the default screen is worth having, btw, in one of the four corners of the device — so that you can in fact just poke at a known point when you inevitably forget to do this.
Off the top of my head, btw, some app types I don't think were mentioned yet are voice recorder, blutooth keyboard or improved soft keyboard software (either to make journaling easier, as well as just email). Possibly something to compress photos to make it faster (cheaper?) to upload, certainly one or more good file manager option. Putting lots of documents on your phone, so for example, that you can look up the perhaps unguessable process for getting your watch to do something obscure (or to get it out of some obscure mode that you don't know how it got into). A simple "burn battery power to make this a flashlight" is a good thing — I used to carry a light LED light as backup, but my phone works fine for those infrequent instances when inevitably the time I decide I need to replace headlamp batteries is in the middle of the night. A kindle or similar free and/or paid first aid book is a really good idea. I'm not trying to be complete here, I'm certain there are more.
I guess another type of "pro tip" might be to keep the d#$m thing functioning on a long trip, and there are IMO two aspects to that (apart from battery power). First is dealing with the inevitable bugs. I've owned and carried into the backcountry enough phones now that I think that a person just needs to expect them to be buggy. And to have worked with the phone before going on a long trip to the point where hopefully not too much will bite you badly. The ones that force a reboot are obvious (and still highly annoying), but it's the subtle ones that can sometimes be really bad. In particular for me, one phone I had did wonderful on-camera panorama photo stitching. In some cases I could tell right on the phone that the software had failed to make a good stitch. But later when home I would then find quite a number of what I thought were beautiful shots where things didn't quite line up right, but looked right on a small camera screen in sunlight.
The other aspect of keeping it working is avoiding physical damage or the like. A nuance to that sometimes is dealing with cold weather, to include maximizing battery life when temps are down around freezing. Hmm, and as an aside to that, let me say that instead of (or in addition to) buying a little conductive thread and stitching it into a light glove finger (so that you can manipulate the screen without removing your glove), an alternative to taking pictures at least — when it's cold enough out that you really don't want to take off mittens or gloves — is to get practiced at stabbing the "shoot photo" soft button with your tongue. I've taken many a shot that way.
But to keep the phone from breaking or being lost, and yet having it readily available, something like a neoprene case that attaches to a chest strap works well. I think it's a mistake to carry a smartphone in a waist-belt pocket, unless you're very consistently careful. In really dusty, gritty environments, keep it in a snack-sized ziplock even when no rain is anticipated (and definitely do that when rain IS anticipated). Some touch-screen functions are just not going to work from inside a ziplock, but when it's in GPS mode I can make it work well enough like that.
Whatever carry case you get for it, it's worth the time/effort/money to get one that fits well. Too tight and it's a frustrating fight at a point when that marmot is poised in just a really cute but temporary way that you want to snap a picture fast. Too loose and it can literally fall out.
On a snowshoe day hike last year, my new phone fell out of an old (too large) neoprene case and sunk deep enough into the snow that when I went back to look for it, it wasn't to be found. So now I've got a case that fits the phone well, AND the little optional carry strap attachment point has a very little ring sticking out of the side that I can optionally clip to a neck cord if I feel nervous.
I also put a rubberized and moderately thick "skin" on the phone. This in turn impacts that neoprene case sizing and adds friction in using the case, but definitely soaks up some modest impacts.
The list of "pro-tips" is pretty large, I guess, really. It's also hard to know what others take for granted, and what's new to at least some. For example, the idea of texting when cell coverage is sketchy rather than trying to call. The idea of having something like skype so that you can make calls in trail towns that have no coverage for your carrier but do offer decent wi-fi. Lots of stuff. Some won't seem like "backcountry" issues to everyone, but when you're on a long hiking trip (one or more resupply stops along the way), the various uses all blend together for the one device.
