I agree with Andre. At one has got to be technique

1. Technique.
Create a solid stance, base of movement and defence.

2. Athleticism
You will get a lot of wrong advice on preparing yourself for boxing. You need to establish the corret methods of training.

3. Nutrition
Two fighters with equal abilities, physicality and preparations meet. The one with the better fuel wins.

4. Mentality
You can prepare your body to go the distance, but your mind is what talks the helm in a fight.
But fighting is easy, its fun, you've got to prepare your mind for the rigours of training which sometimes seem fruitless