First, by having just about nothing running. Use a boot manager and turn off everything the phone can keep running without (including Google Play services and all the radios - Bluetooth, wifi, 2G [talk and text], mobile data and GPS).
Second, condition your battery when it's new. (If it's not new, and you used it when it wasn't conditioned [there's no way to tell, which is why it's wise to condition all new batteries], the time between charges is already reduced.)
People get those numbers by running the phone until there's just about no charge left in the battery - a good way to need a new battery every 6 months. Since the battery isn't user-removable, that's not such a great idea. (I'd never buy a phone like that. If I need more time than I can get by using the battery properly [discharging it to no less than about 40%], I carry a spare battery - or 2 or 3, depending on the situation. Good lithium batteries are pretty cheap these days.)
