Items should go into whichever bag is open by default. Then the user can maintain their own bags without having to go through a sorting process after a mining/gathering foray.
Intelligent sorting would improve even more. Set the bags default to mining, foodise, hootch (don't you have a bag full of hooch? wha!), groceries, produce, gems etc.
Additionally, items go to the first open slot in your "home row" if there is one, and then to the first open slot in the bag furthest to the left in your home row. When using inventory (grinding, cooking, etc) it takes from the first slot it finds, so first from the home row, then the first one in the bag furthest to the left in your home row.
With this in mind things I do to maintain my bag organization:
Slot one is my toolbox. This prevents most items from ending up in here accidentally.
Slot two is empty. This means that random drops and so on mostly go to this location, where I can drag them to the appropriate bag. It is also a free slot so that I can pick up bags at home to rifle through them without having to drop another bag. The third slot has a bag where I expect that "overflow" items will end up if I pick up a lot of stuff all at once.
The rest of the bags are organized idiosyncratically, but what may be of interest to you is the idea of "pre-loading" certain bags. For instance, when my sparkly bag is otherwise empty, I fill each slot with one sparkly by first, dropping one sparkly rock, then I drag-and-drop it to a free slot in the bag. This serves two purposes: it makes it impossible for a random drop or a re-organized item (bundled grain, items returned to my inventory because the action was unsuccessful) to end up in this bag, and it also ensures that when I am mining sparkly, all the sparkly goes to the bag without any further intervention. The process is a little time consuming, but it works. Other items I pre-load: metal ore, metal ingots (changing to moly ingots as the night goes on), meat, milk and grain, tree harvests, produce, etc.
It has an added bonus that, when sparkly shows up in my free space, I know the bag is full and it's time to grind.
I'm not saying that the Bag UI isn't problematic, but until there is a major change to this, I wouldn't hold my breath for any fixes. This keeps me sane in the meantime.
That's a nice trick Scarlett. I might use that myself.
But still +1ing Disco Bob's idea. It's nice and simple and (should be) easy to implement.
Instead of going to your "home row" first, it first finds empty spots (left-to-right) in any open bag, then tries the home row (if no bag is open or the open bag is full), then does bags from left to right as usual.
Just one thing I want to add to this since it's related: A very nice interface change would be, when dragging an item onto a different, non-bag item, it would be quite helpful if the items swapped places. This would be extremely useful to us packrats whose bags are usually brimming with items to begin with, and would probably be convenient to others as well.