A dictionary of Confusable Phrases: More Than 10,000 Idioms and Collocations

don’t sweat it • don’t sweat the small stuff
don’t sweat it—(U.S. sl.) don’t worry: Don’t sweat it! We’ve
got plenty of time to get there before the show starts.
Note: 
The expression does not correlate in meaning with the
phrase no sweat—(sl.) used to say that one can do smth. with no
difficulty or problem: “Do you think you’ll be able to manage
all those boxes yourself?” “Yeah, no sweat!”
don’t sweat the small stuff —(U.S. sl.) don’t worry about
things that are not important: You may have heard the phrase
don’t sweat the small stuff. The person that said that wasn’t
selling his home.
don’t tell me • say no more
don’t tell me —(also: never tell me) is used to express in-
credulity, disappointment, or dismay: “Don’t tell me we’ve got
to go through that again,” said one executive member in an
audible groan.
say no more —you need not add anything (said when in the
course of a conversation one party communicates his total under-
standing of smth.): I hadn’t thought much about this post, ac-
tually, as a piece of writing. But say no more: I know well
enough not to argue with such a lovely woman.
Note: 
The expression does not correlate in meaning with the
phrase it goes without saying— it is too obvious or well known:
It goes without saying that a good first impression is critical.
See also: don’t mention it / don’t talk to me about it.
don’t you forget it • forget it
don’t you forget it—an admonitory phrase used after a state-
ment to emphasize the idea: If it wasn’t for them, we wouldn’t
have all this, and don’t you forget it.
forget it—(imper.) 
1
. take no more notice of it; don’t mention
it: “What do I owe you for lunch?” “Forget it.” 2. don’t even
think about it: If you’re expecting a description of what those
thugs did to us, please forget it.