Most people cook well and pack badly. A bento has to survive a bag, a bicycle and three hours at room temperature, and that is a different problem to getting dinner onto a plate.
Start with rice as the base and pack it firmly. Loose rice shifts, and once it shifts everything else follows. Fill the box properly, because empty space is what lets food move around and turn into that single sad pile.
Keep wet things away from dry things. Anything with a sauce goes in its own cup or gets drained before it is packed. A crisp piece of katsu will go soft in minutes if it is sitting next to something dressed.
Let everything cool completely before the lid goes on. Trapped steam is the quickest way to ruin texture, and a warm sealed box is not somewhere you want food sitting for hours.
Colour is the last step and it is not just decoration. If you have three colours in the box you have almost certainly got a reasonable spread of vegetables, protein and carbohydrate without having to think about it.