My grandmother taught me how to make a good (delicious and nice-looking) obento.
The most important thing she said is you should not only use stuff of the same color in it. If you put some pieces of grilled chicken, it's brown and if you put some yakisoba (fried noodle) beside it, it's brown too. Then the obento will not look very delicious no matter how it really is so. You should put something red, green, yellow instead of two brown stuff, it would look far better.
red stuff can be tomato, umeboshi (pickled plum), benishouga
green, broccoli, parsley, green peas, kaiwaredaikon(radish sprouts)
yellow, lemon, tamago-yaki, takuwan
brown, hamburger, meat ball, croquette
Please look again at all those mouth-watering obento photos bezz-san posted, they are all so colorful! If your obento has all those colors, then really it looks nice and also has very well balanced nutrition.