OK, so you read the part about "it's only to be installed for development purposes" and figured that was the dead hand of Steve reaching up to keep you from having fun right?
Unfortunately: it wasn't.
Right now they aren't kidding and you need to understand something to use iOS 9 beta as your daily driver: a development device has the beta installed and a development app. It doesn't have 1024 apps and 100GB of it's 128GB of storage filled up with movies, photos etc... nor is it plugged into every social network and email account known toThe Cloud. And it's chock full of bugs that developers don't care about because they're focused on making siri work with their app.
Got it?
So if you still want to make a daily driver out of iOS 9 beta, because you know, you are wink-wink-nudge-nudge testing the bits to help find bugs for Apple instead of fixing up your own code....
For the first day you will need to treat it like a WIndows OS Desktop.
Keep it plugged into power. (Desktop)
Keep rebooting it. (Windows OS)
Ideally you should factory restore to get on iOS 9 beta the first time. This gets you a fresh clean install and a stable base to load your 100GB of "test data" onto.
So back everything up and then do the alt-restore to load the beta onto a nice scrubbed clean device.
During activation choose to restore from that back up. Apple generally puts more care into this path than the upgrade device path because this is how developers roll: fresh device, restore test data, install their app for testing. So roll like a devloper for the best experience for yourself.
When Apple starts offering over the air upgrades you can rest assured that means Apple has put more time into the on device upgrade experience. Developers will be OTA'ing at this point to not disturb their test environments for their apps so you are following the developer track.
After restoring and activating: reboot.
Yeah I know, it rebooted itself already: reboot. See abouve about Windows OS. Lots of beta bits for onboard feature discovery etc.. Why couldn't it find Wifi? I don't know, that's why they call them buggy beta bits, get it? It needs several passes through the init scripts to get the hardware shaken down.
After that your apps will load out of the store and your 100GB of "test data" will be getting looked over. You will notice your device is much warmer than usual as the various bits of that 100GB of data get poked and prodded. You will want to be plugged in the whole time and you will want to reboot every three hours or so to keep things turning over on all that data. You can try things out but keep in mind that most stuff will take hours to stabilize eg. photos, movies, email, twitter are all being poked and prodded to implement things like the new siri features. Don't plan on using siri? Too bad, you still have to put up with all her poking and prodding until she's done.
After about 24 hours of this things will get stable enough the battery might not immediately plunge when you unplug. Might. Some folks notice iOS 9 Beta self reports on the Home Screen being a power hog. I myself have noticed the Mail app and the Home Screen are in a steel cage match to see whom can use the most power. This is precisely why I have a 3,300ma Lip Stick Charger: for iOS betas. :-)
Why 24 hours? Developer devices don't have siri and friends needing to poke and prod 100GB of personal data, every social network and email service known to The Cloud. Beta bits are not fast bits and developers don't care because they didn't load 100GB of personal data and plug in every social network and email service known to The Cloud. So pull up your developer pants and don't go whining on the Beta Forums about how if Apple don't fix it quick your're going back to iOS 9.4 beta: just do it now before something eats your 100GB of personal data.
I recommend daily back ups and reboots until beta 2.
Bugs bang up things like configurations and the init scripts during a reboot sometimes kiss the boo boo's and make them feel better. And developers' apps are crashing their devices for them, they get lots of reboots: you need reboots too. Roll like a devleoper.
Yes, that was your device crashing and rebooting when you asked Siri for "Reminders". There are bugs ready to crash your device at unexpected moments like a daily double popping up in Jeopardy!. But this is why you're here right? You are helping Apple test iOS 9 Beta. Every time you hit a Daily Double make sure you file a bug report OK?
Have fun and be careful out there amongst the bugs.