For example, take a look at the following sentence:
There’s nothing to see — no one can handle you, dear.
What are the general guidelines?
Dashes, in the context that you seem to be using, are used for (emphasized) parenthetical clauses--much like the two prior, but with special emphasis drawn to them. As demonstrated, parentheses are used for parenthetical clauses with diminished emphasis, and commas for parenthetical clauses--and appositives--with more-or-less equal emphasis with the rest of the statement.
Colons, on the other hand, are used: when building a list, to separate parts of a title, in the salutation of formal letters, and occasionally to introduce quotations.