1. I'm still relatively early in the game (haven't gotten to any petroleum processing). I probably should advance... but I'm having fun with this. Restarted the game on my laptop because I was away from my desktop. My laptop is able to play this game just fine (woot woot).
2. I created a "central storage" bank of boxes (they'll have to be "smart chests" in 0.12... but all chests are "smart" in 0.13, so I just use wooden chests). EVERY item gets its own chest and a filter-inserter to ensure that the chest remains purely of that item. All chests are wired to the "Green" network (all items in green network are added together).
3. I have a "filter belt" which feeds the central storage inserters. The filter-belt is a large loop, so if an inserter "misses" an item, it will eventually loop around and get a 2nd (or 3rd, or 4th) chance to pick it up.
4. To ensure the "filter belt" continues to move, items are ONLY inserted into the filter belt if (Green Network: Item < 0). I hook up a series of green "Constant Combinators" that set negative values for specific items. (For example: if I want the network of smart-chests to have 1500 copper, I set the constant-combinator to -1500 copper. Up to 15 different items can be requested per constant combinator).
5. I have a "Output network", consisting of Red Wires. The idea is to have an ideal "slush" of mixed output on the singular belt. This does work out, but in practice my current design is very slow. I'm currently thinking of improvements. Some basics: Every 4 belts are connected to a 2:1 joiner to ensure that the "slush" doesn't block all of the other inserters. Ex: If I set the Red-network combinator to -15 copper, -20 iron plates, -15 gears, and -15 green circuits: the red-wire inserters (which are removing from the "smart storage") will continuously output. This uses the 0.13 feature where you can hook up red (or green) wire to a belt to count the number of items on the belt.
6. Aside from Iron, Copper, Green Circuit, Gears, and Wire production, ALL of my automated production is in this "output network" ring. "Well-mixed Slush" on the right-side of the belt, "Factory output" on the left-side of the belt. Science labs are positioned on the "left" side of the belt (since they have no output, they'll never have to worry about the "input-slush" blocking them), while all factories are on the right side (to ensure that the "output" lane for all factories remains open). The only way a factory "shuts down" is when their associated "out inserter" (from the green network) detects an overabundance of a particular item.
The output network works pretty well, but my current design is redundant and hard to setup. You need to set ALL of the belts to be on the red-network, then set them to "count", then disable the "enable/disable" switch. Its relatively slow and takes a lot of twerking to make the "output mix" just right for optimal performance. Even then, I find myself feeding the factories manually a lot of the time.
But the "filter belt" and "central storage" bank are extremely useful, especially since I managed to get it "blocking". When the chests fill up with 1500 Iron (my current set amount), the iron processing "backs up" and eventually automatically shuts down. It becomes very easy to see which items I have in excess, which items I have "excess storage" (but not excess production).
The factories connected on the "output network" have proven to be extremely flexible as well. By changing the smart-inserter "out" to match the factory output and connect it to the green network (so that I satisfy condition #4 stated above), factories automatically shut down when "central storage" says that an excess of a particular item has been produced.\
I think the design of the "central storage" and "filter loop" are solid, and I'll be constructing this every game from here on out. Its just super-useful to have one location keep track of all of your excess output.
Long story short: I figured out how to use red-wire / green-wire circuits to make "long-distance" backpressure. My long-term hope is to simplify down a lot of factory design to use fewer transport belts, and instead use a single "mixed" transport belt for a lot of demand.
Everything else I said was noob stuff. I looked around the factorio forums and found a ton of simpler designs that did what I wanted. Two fast inserters (possibly one of them a filter-inserter) and two transport belts is all you really need to create a "on-demand buffer"...