You should wear your avalanche transceiver under your outerwear (preferably on top of your base layer). This will reduce the likelihood that it will be damaged if you slam into trees or rocks, and more importantly, it will ensure that you don't need to take off your transceiver when you change layers while in avalanche terrain (just as you don't want to take off your seatbelt while driving).
When you put your transceiver into its harness, face the device's screen toward your body to minimize the chance of it being damaged by obstacles. You should also position the transceiver in its harness so the little blinking light that indicates it is turn on is visible (almost all transceivers have a light that serves this purpose).
You can keep your transceiver in a well-attached pants pocket in lieu of the transceiver's harness, although it might be more susceptible to impact damage. If you do choose to use a pants pocket, make sure it is the only thing in the pocket. Even small foil food wrappers, like energy gels, can interfere with the transceiver's signal. And be sure to position the screen toward your body when it is in your pocket.
A good rule of thumb is to "turn it on when you put it on and turn it off when you take it off." Making sure you never wear your transceiver when it isn't turned on will ensure you that you never enter the backcountry with it turned off. A slightly less stringent rule of thumb is "on in the car and off in the bar." Avalanche transceiver batteries typically last 200 to 300 hours—don't be a cheapskate by trying to conserve your batteries.
The transceiver should be securely attached to its harness or your clothing with a leash. The leash might keep the transceiver near you in the extremely unlikely chance that a second avalanche occurs when you are searching, but more importantly, it will keep your transceiver from sliding down the mountain if you drop it.
It is preferable to turn off your other electronic gear to avoid interference with your transceiver. If you do keep other electronics turned on, they should be at least 8 inches (20 cm) from your transceiver when you are transmitting and at least 20 inches (50 cm) when searching. Transceivers are much more susceptible to interference when in search mode. And although airplane mode isn't perfect, it is much better than letting your phone converse with cell towers while you're searching for your friend.