The five principles of traditional Japanese cooking, which are also applied to bento, are extremely useful in putting together a meal that has a variety of colours (visual stimulation) and textures (sensory stimulation in your mouth), different nutrients (which foods of different colours are an indication of), a range of cooking styles and a the five tastes classified in Traditional Chinese Medicine (salty, sweet, sour, bitter and spicy/pungent) which help to ensure a meal balanced in yin/yang as well as other as oriental medicine concepts.
Although it may not always be possible to fulfil these criteria, paying some attention to them will certainly pay off. Again, the wider your repertoire of ingredients, cooking methods and flavours, the more likely you will be able to come up with a combination that comes close to creating the variety of colours, textures and tastes that make an appealing bento.
More bento tips:
Bento primer part 1: foods for bento