What are the usage rules for choosing among commas, parentheses, and em-dashes for setting off parentheticals in a sentence?

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I use a lot of parentheticals, so I suppose it is high time I figure this out.

english

grammar

other

factual

tynamite

Commas are used to separate parts of a sentence, to make it easier to read aloud by providing a rest, and to break the parts up, so that they make logical sense.

The panda eats, shoots, and leaves. (the panda is a murderer)
The panda eats shoots, and leaves. (the panda is peaceful)

Parenthesis, curved brackets, are used to provide additional information to the sentence, which provides further knowledge to the ill-informed, or to provide a diversion to go off on a new tangent.

Alexis threw her food in the bin (as she didn't eat meat), and then got up to leave the room.

There are many ways in which politicians could curb inflation, which is continually devaluing our currency as we know it. In addition to a 99p coin being created, the 1p coin being scrapped would greatly improve the economy (as 1p coins cost more to produce than they're worth).

Dashes are the wildcard, rarely used; which can be used for one of the following reasons.

In direct speech, to show that a speaker has been interrupted.
To provide the same function as parenthesis mid-sentence, but provide a much longer pause.
To work as a conjunction towards the end of the sentence, so that a statement of a sentence can be suffixed to the end.

"Can I have some of your-"
"No."

The lights went out in the house - a fuse needed replacing - but nobody knew that.

No I don't want to do that. It never crossed my mind - unless it's what you want.
All was fine - for now.