I usually make it that morning, but I have pre-prepared stuff that makes it quicker to assemble: grilled chicken, pre-washed fruit, pre-cut vegetables. If I know I will be short of time the next morning I put the fruit and vegetables in the box the night before. If you close the box and put it in the fridge, even cut apples don't turn brown. Usually it doesn't take me more than 10 minutes, I do it while I'm waiting for the coffee to boil and my oatmeal to warm up in the microwave. That's why I tend to do simple things like wraps and not cooked food, although I will use cooked leftovers sometimes.
Mini-quiches I can cook pretty quickly in my countertop oven, so sometimes they are fresh. The sushi is also made fresh in the morning, but with re-heated, pre-cooked rice frozen in portioned ziploc bags -- I learned that from the
lunchinabox web site. (Bag the rice while it is still hot from the pot and it won't dry out.) Biggie at lunchinabox has lots of good tips for efficiency. You also have to keep in mind that if you cook something that morning, it has to cool off completely before you put it in the closed box, or it will steam up and get everything damp and mushy.
I have three or four different boxes that I use, between 500ml and 700ml size. The size is useful to keep in mind for portion control -- it may look really small but you can pack a lot in there. If you want to put it in the dishwasher keep in mind that many of the real Japanese bento boxes need to be handwashed or they can warp.
I like my two metal boxes, and if there is anything that might be leaky, I use my lock 'n' lock plastic boxes. They all have extra locking mechanisms because I had the tops fall off a couple of times on regular plastic boxes. I just got them from dollar and import stores around here. (There are a lot of Indian/Asian import markets in our town.) I did buy some silicone divider cups but I hardly ever use them, I just jam everything in together. Well, for freezing things in portions they are handy. If I want to divide or separate something like a hardboiled egg, I use a lettuce leaf. I also have some tiny plastic bottles for soy sauce, olive oil, etc. that I got in the Daiso Japanese import store, and some small plastic screwtop jars for salad dressing. If I start bringing yogurt I will put it in one of those. And I steal ketchup and salt and pepper packets from fast-food joints.