Some of the tricks are not precisely about "latexing_with_emacs", but might be for general latexing, general editing with emacs, or even general unix/linux.

In general, the tips are ordered starting with simpler and moving on to the more arcane.

• While typing, run latex early, run latex often, after every couple of lines of typing. It is much easier to spot latex mistakes this way, and saves the time needed to wade through large amounts of latex trying to weed out the mismatch or other mistake.

• The Most Useful Shortcut!!
  For those people who run latex by going to a shell, and running a "latex myfile.tex" command, I am going to make your life much faster... learn the sequence
  C-x C-s C-c C-f C-x 1
  C-x C-s saves the file, C-c C-f runs latex on it, opening a split window in the process, and C-x 1 unsplits back to a single window hiding the "tex-shell" buffer.

• Viewing the document: dvi & postscript.
  I recently found a colleague running dvips after each latexing, and then opening the postscript file. Waste of time!
  Instead, keep the dvi file open using xdvi. Each time you re-latex the latex file, the dvi display updates itself!
  Almost WYSIWYG!!

• Switching between windows.
  Even if you did away with the need to go to a shell/terminal using the habits mentioned in the last two tips, you still have to switch back and forth between the xdvi window and the emacs window. Do you use the mouse for switching between windows?
  On most modern window managers, ALT-TAB will cycle between windows. This works in KDE, Gnome, and also for many of the simpler no-frills window mangers, like icewm, my personal favorite for the last year or so.
  Why waste energy moving your hand away from the keyboard? :-)

• The AMS-LaTeX packages.
  If you latex a fair amount of mathematical stuff, but are not using the AMS math packages, something is definitely wrong.
  The AMS "short math guide" needs to be handy; you can get it at the AMS short-guide page.
  It might also be worthwhile to browse through the more general AMS latex pages .
  Being a thoroughly modern person in terms of typesetting :-) I now always have \usepackage{amsmath,amssymb} immediately after my beginning \documentclass{}.

• Still using \begin{eqnarray} ... \end{eqnarray}?
  The AMS-LaTeX guide discourages using the eqnarray environment, because it is so ugly and unwieldy! Times have changed!!
  Instead, use one of the AMS environments: multline, split, gather, align. Whichever is suitable to your multiline equation(s). And any of these will look much better than whatever you were going to produce with eqnarray!

• MULTILINE or MULTLINE?
  Common mistake when you first start using the AMS packages. For some reason their environment is called multline, without an "i" after the "t".

• Justifying text.
  In most major modes of emacs, the paragraph (on which your cursor is) can be justified using
  M-q

• Do shortcut definitions for \begin{equation}, \end{equation}, make sense any more?

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• Bored at having to type \end{*%*}?
  If you've already typed \begin{*%*}, where *%* is any environment, use
  C-c C-e and emacs will put in the \end{*%*} for you. Isn't that brilliant?

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• Tired of \begin{*%*} also?
  Check out
  C-c C-o

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• GNU Emacs Reference Card. [Emacs.]
  If you don't have the GNU Emacs Reference Card printed and pasted on the wall next to your terminal/machine, maybe you should. It's a great way to learn one more new keyboard shortcut each month, when you have last month's shortcut already internalized into a habit.

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• Tired of .aux, .tex~, and .log files?
  

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• Syntax highlighting.
  You might find that you like your latex syntax highlighted in color. (I don't personally find it very useful, but you might, in catching syntax mistakes or as eye-candy.)
  For some perverse reason, the syntax-highlighted mode in emacs is known as "global-font-lock-mode". You can do
  M-x global-font-lock-mode to toggle between syntax hilite & non-hilite modes.
  (I hope you know to tab for command completion rather than type in all of global-f... :-)
  For mouse-lovers, Help -> Options -> Global-Font-Lock, on the menu.

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• Switching Between Buffers. [Emacs.]
  

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• Including eps files Inline
  Like all sensible people, I usually use the graphicx package for including eps files, but there was one case I had to use the epsfig package. This was to include eps files inline to form one equation having several graphics elements. Maybe there is some way to do this using graphicx also, but I don't know it.