DO NOT:

Fly in restricted areas: This includes public buildings, state parks (US), airports, military installations or a DJI no-fly zone.
Fly at public events: Games, concerts - anywhere where there is a crowd
Fly alone: Always use a spotter, which is in many countries required by law
Fly near: Power lines (which can interfere with the electronics or in the worst case produce a short leading to a major power outage), glass or metal (which can reflect and/or interfere your signal), open water, animals (some animals gets very distressed by flying objects, which could even lead to miscarriages)
Fly in your back yard, if your yard is not at least 300ftx300ft (100mx100m). I know, it's very tempting to get started, but rather do a trip to a nearby field. It's much more rewarding and forgiving. You can take pictures and video the roof of your house soon enough!
Fly far or high: Again, this is very tempting to rely on the apps and the electronics and push the limits, but don't do it, if you don't know the basics first. It is also forbidden by law in some countries to fly far and high.
Fly outside the line of sight: If you can't see your drone, it may not be able to see you. Just like with a rear mirror of a truck.

DO:

Practise: It is generally best to practice with a cheaper mini drone, such as a Hubsan X4 (which costs less than 1/40 of a Phantom) and is really good to fly indoors, to get a feel for the controls and aerodynamics. Also a lot cheaper, if you crash.
Inform yourself: If you're flying with a smartphone and tablet app, know your limitations and options. You still will need to know how to fly manually and unassisted. Inform yourself on how to make the best use of resources on your smartphone/tablet - if you don't know how your smartphone/tablet works, then chances are you won't have a very good experience and "random" app crashes. Do you really know how a radio communications work? How a Wifi network works (for Standard)? What could cause interference and problems?
Be aware of the consequences: What could happen right at this moment, if my drone falls out of the sky? I am ready to pay the price for any damages, injuries or loss of life? Do I really want to risk losing this expensive device (or even worse, deal with DJI support)?
Be cautious: Not every firmware or app update will immediately make things better or solve all your problems or make up for a lack of piloting skills. A wrong firmware update can potentially brick your Phantom. Wait for a few days/week before upgrading and study the forums for positive and negative feedback.

Any probably most importantly:
Don't rely on the electronics: Batteries can fail, GPS can fail, the app could fail, a prop can break - can you recover the drone in an emergency situation?

Hope this helps.

DO NOT:

Fly in restricted areas: This includes public buildings, state parks (US), airports, military installations or a DJI no-fly zone.
Fly at public events: Games, concerts - anywhere where there is a crowd
Fly alone: Always use a spotter, which is in many countries required by law
Fly near: Power lines (which can interfere with the electronics or in the worst case produce a short leading to a major power outage), glass or metal (which can reflect and/or interfere your signal), open water, animals (some animals gets very distressed by flying objects, which could even lead to miscarriages)
Fly in your back yard, if your yard is not at least 300ftx300ft (100mx100m). I know, it's very tempting to get started, but rather do a trip to a nearby field. It's much more rewarding and forgiving. You can take pictures and video the roof of your house soon enough!
Fly far or high: Again, this is very tempting to rely on the apps and the electronics and push the limits, but don't do it, if you don't know the basics first. It is also forbidden by law in some countries to fly far and high.
Fly outside the line of sight: If you can't see your drone, it may not be able to see you. Just like with a rear mirror of a truck.

DO:

Practise: It is generally best to practice with a cheaper mini drone, such as a Hubsan X4 (which costs less than 1/40 of a Phantom) and is really good to fly indoors, to get a feel for the controls and aerodynamics. Also a lot cheaper, if you crash.
Inform yourself: If you're flying with a smartphone and tablet app, know your limitations and options. You still will need to know how to fly manually and unassisted. Inform yourself on how to make the best use of resources on your smartphone/tablet - if you don't know how your smartphone/tablet works, then chances are you won't have a very good experience and "random" app crashes. Do you really know how a radio communications work? How a Wifi network works (for Standard)? What could cause interference and problems?
Be aware of the consequences: What could happen right at this moment, if my drone falls out of the sky? I am ready to pay the price for any damages, injuries or loss of life? Do I really want to risk losing this expensive device (or even worse, deal with DJI support)?
Be cautious: Not every firmware or app update will immediately make things better or solve all your problems or make up for a lack of piloting skills. A wrong firmware update can potentially brick your Phantom. Wait for a few days/week before upgrading and study the forums for positive and negative feedback.

Any probably most importantly:
Don't rely on the electronics: Batteries can fail, GPS can fail, the app could fail, a prop can break - can you recover the drone in an emergency situation?

Hope this helps.