Q. I can’t get a definite answer on how to punctuate a sentence that starts with “trust me.” For example, “Trust me, you don’t want to do that.” Would this be considered a comma splice? Would it be better to use a period or em dash, or is the comma okay? What about “believe me” or “I swear”?
A. Any phrase like “trust me” at the beginning of a sentence that is roughly equivalent to a “yes” or a “no” can normally be followed by a comma (as covered in CMOS 6.34):
No, you don’t want to do that.
is like
Trust me, you don’t want to do that.
whereas
Yes, I’ve edited the whole document.
is like
Believe me, I’ve edited the whole document.
and
I swear, I’ve edited the whole document.
You could use a stronger mark of punctuation for extra emphasis:
I swear! It’s not a comma splice!
or
I swear—it’s not a comma splice.
among other possibilities
But a simple comma will be the best choice in most contexts (and won’t get you in trouble for using a comma splice—at least not with us).