During the week I often get so rushed and busy with everyday life that I barely have time to stop and think about anything, including making bento lunches. So I try to do a little prepping over the weekend, when I have some extra time. I'm not really talking about spending hours in the kitchen, but easy things that can be done either in a few minutes, or unattended while I do the laundry or just take a long nap.

Shopping

I do most of my grocery shopping on the weekend though I do buy veggies and things on other days. (Our local supermarkets tend to start their sales on Wednesdays or Thursdays, so that affects the shopping pattern too.)

• I also consider vegetables like green onions, round onions, garlic, ginger, carrots, celery and chili peppers to be staples and I get nervous without them in the fridge. Otherwise, I try to buy whatever fruit and vegetables are in season and try to plan the bentos around that.
• Other fresh things I stock up on: any chicken, meat or fish products that I can use, especially when they're on sale; various kinds of tofu or other vegetable proteins; eggs. Any extra meat or fish goes to the freezer. (Tofu can be frozen but will change texture, so usually I just buy what I'll be using during the week.)
• I do a general check of stock ingredients and replenish - beans, rice and other grains, spices and condiments, flours and so on; frozen peas, edamame, natto etc. for the freezer.

Prepping food

I don't do all of these things every weekend, but I do often do one or two of them. I think the key to keeping habits going is to not try to do everything.

• Leafy greens, especially spinach and lettuce, can be a hassle to wash, so I try to wash them soon after getting them home. Spin them dry in a salad spinner and store wrapped in paper towels and then in plastic bags - they stay fairly fresh for at least a few days. (You can try pre-washed and bagged leafy greens if you can be sure they are safe.)
• I soak some kombu seaweed and either dried shiitake mushrooms or bonito flakes in cold water in a big jug (as in the photo above). The soaking liquid can be used as dashi stock, and the shiitake can be used for various dishes. The kombu can be reused once (though it does lose flavor). After that I usually cut it up and use it in soup, or freeze it and eventually use it to make tsukdani (recipe to follow).
• Besides freezing rice, I often make a batch of beans of some kind - soy beans, kidney beans, azuki beans, etc. I freeze the cooked beans with a little of the cooking liquid in 1 to 2-cup portions. You can use canned beans, but the variety of dried beans is a lot wider usually than canned. And it's way cheaper. For making vegetarian dishes this is sort of required I think.
• I make one or two joubisai (staples) like salted salmon and stock them in the freezer, or a big pot of something like stewed potatoes and meat (nikujaga) for the fridge. I usually do this while cooking dinner - for example we might have some of the nikujaga for Sunday dinner, and the rest in some guise in bentos during the week.

What I don't do much of

• Pre-chop or pre-grate onions, ginger and garlic. Some books recommend you do this (even making little packets of frozen onions or grated ginger). I would rather use fresh ginger and so on whenever I need it.
• Pre-cook vegetables more than a day in advance. I think they lose freshness, flavor and texture if kept in the fridge for too long. I do sometimes cook things the night before.

Even if you can manage to do one or two things to be ready, your bento making will go a lot quicker and smoother, and you won't be as liable to giving up and grabbing a Big Mac Meal!