Re: To Comma or not to Comma
It strikes me that we're all doing this the hard way. We're taking a complicated sentence and trying to break it down into its component clauses. It would be far easier, I think, to take a simple sentence and to build upon it. So let us put the sample sentence in the simplest possible language (the Dick and Jane version, if you like). Let's change the sentence:
"I was walking down the street, and, just as I turned the corner, there was a loud crash."
to
"I walked down the street. I turned the corner. I heard a crash."
I've changed the wording. In my opinion, the awkwardness of the original comes, not from the commas, but from the passive voice and the useless adjective. A loud crash? Give me a break. Have you ever heard a quiet crash? When I line edit, I delete all adverbs and adjectives that don't pull their weight. It could be that you want this sort of casual voice for your first person narrator, but I wouldn't. It sounds too much like all the other slush on the slush pile.
Now we have a simple sentence to work with, we can see how we might use commas. There are really only three uses. First, we use commas to separate the elements of a list (and like others here, I prefer the American style, which is clearer). Second, we can use a comma and conjunction to join two simple sentences together and create a compound sentence. Our example becomes:
"I walked down the street, I turned the corner, and I heard a crash."
I would probably further simplify this by dropping I from the second two clauses:
"I walked down the street, turned the corner, and heard a crash."
I feel this is superior to the original version of the sentence. It has a distinct rhythm. The original has the rhythm of a drunk, white man dragged unwittingly onto the dance-floor by a girlfriend.
Third, we can use a comma to separate a subordinate or adjective clause from the main clause of a sentence. So our example might become:
"Walking down the street, I turned the corner and heard a crash."
Here I'm using the rule that you can drop a comma from a compound sentence if the clauses are short. We don't need to write: "...I turned the corner, and heard a crash."
So, that's about it. I recommend that, if you have trouble, you should start with a simple sentence and build it up, rather than start with a complicated sentence and try to break it down. It works for me. Beware, though, I'm talking here about line editing. For a first draft, I put all the fussy grammar and punctuation aside and concentrate on finishing.