These are mainly for copy editors, but if you care about making your
documents look good, you may find these useful.

0. this should be too obvious to write, but some people miss it: use
TeX to typeset all your math.  For example, write $x>3$, not x$>$3 or
some other abomination.

1. when using a binary operator such as \sim, put the whole relation
inside the math, e.g., $z\sim3$, not z$\sim$3.

2. when using what TeX thinks is a binary operator as a unary
operator, _either_ a) put the operator in math by itself (e.g.,
'$sim$90~second'), _or_ b) put {} around the operator as in
'${\sim}90$~second'.

3. use 'e.g.,' and 'i.e.,' -- two periods and a comma -- when using
those abbreviations.  In the old days, the expressions would have
been in italics.  A few publications may still do that, but I think
most don't.

4. use \arcmin, \arcsec, \micron, \farcs, and others defined by the
journal .sty files when appropriate.  A particularly useful one is
\ion.  For example, 'H II' is correctly formated by \ion{H}{2}.
(That's for AASTeX.  Some journals would use \ion{H}{ii} instead.)

5. use ~ when you want a non-breaking space such as between a value
and its units, e.g., 90~seconds or 90~s.  If the value needs math
mode, the ~ and units go after the closing $, e.g., '$10^{33}$~W'.

6. There are four kinds of horizontal-line punctuation in TeX:

_hyphen_ has many uses but most often combines two words that serve
the grammatical function of one: star-forming galaxies

_en dash_ denotes ranges or relations: color--magnitude diagram, 2--4

_em dash_ is like a comma but implies the subordinate clause is more
important than the main text.  (Parentheses do the opposite, and
actual commas imply equivalent importance.): ... stars---the Sun---

None of the above has spaces before or after the punctuation.

_minus sign_ is used for math: $a-b$ or J1234$-$456

7. this one is really picky, but a sentence-ending period should have
extra space after it.  TeX usually does that automatically, but it
fails when a sentence ends with a capital letter ("... SMBH.").  In
that case, insert \null before the period, "... SMBH\null."  As an
alternative, you can make _all_ periods into sentence-ending ones by
putting the commands below into your document preamble.  The comments
explain further.  The opposite case is more common: you need to mark
a period as non-sentence-ending.  For that, just put ~ after the
period for a non-breaking space or '\ ' (backslash then space, no
quote marks) for a break-allowing space.

As I say, this is all stuff for copy editors, but sometimes you want
to produce a document that looks good when you don't have a copy
editor handy, so it may be useful some day.

-----for TeX file preamble-----

%   PUNCTUATION SPACING
%  By default, punctuation [.?!:;,] is followed by extra space EXCEPT
%  when the punctuation follows an upper case letter.  The following
%  removes the exception, i.e., punctuation will produce extra space
%  regardless of what character precedes the punctuation.  If you
%  don't want the extra space, follow the offending punctuation mark
%  with '\ ' or '~'.  \frenchspacing and \nonfrenchspacing work as
%  usual to turn extra spacing off and back on, respectively.

\sfcode`A=1000 \sfcode`B=1000 \sfcode`C=1000 \sfcode`D=1000
\sfcode`E=1000 \sfcode`F=1000 \sfcode`G=1000 \sfcode`H=1000
\sfcode`I=1000 \sfcode`J=1000 \sfcode`K=1000 \sfcode`L=1000
\sfcode`M=1000 \sfcode`N=1000 \sfcode`O=1000 \sfcode`P=1000
\sfcode`Q=1000 \sfcode`R=1000 \sfcode`S=1000 \sfcode`T=1000
\sfcode`U=1000 \sfcode`V=1000 \sfcode`W=1000 \sfcode`X=1000
\sfcode`Y=1000 \sfcode`Z=1000