* Moving Soundfiles:  To move a soundfile to a different place on the
time line, select the track then put the cursor where you would like the
file to play.  Then, select the menu item "Project -> Align Tracks ->
Align with Cursor".

* Non-destructive volume changes:  Select the volume handle tool from
the toolbar at the top left.  It is the one with the funny white
triangles and blue line.  With this tool, you can "shrink" and "grow"
the height of a file, which corresponds to its amplitude.  Click once to
create a break point.

* Select all in one track:  There isn't an easy way to select everything
in one track.  What I do is click near the beginning of the track, hit
the menu item "Edit -> Select -> Cursor to End", then hold down the
shift key (which allows you to change the selection without
unselecting), and drag the left edge of the selection over to the end of
the file.

* To add an audio file to an existing project, use the "Project ->
Import Audio", not "File->open".

* A weirdness of the interface is it lets you do almost nothing while
sound is playing.  All of the menu items are faded out.  Occasionally,
on my version (this may have changed in the latest), it doesn't seem to
register properly that audio has stopped playing, and it doesn't let you
do anything.  I find that clicking the play button several times and
then clicking stop when it finally starts playing solves that.

* It has a tendency to crash occasionally (what great software
doesn't?).  Save often, as there isn't (yet) an automatic recovery
facility.  Since most of the edits that are interesting in audacity are
destructive (that is, they physically change the audio data), and "undo"
functionality is lost in a crash, when working on an important project I
will sometimes save multiple copies of the project, and delete the older
ones when I know I don't want to undo that far.  That way, I can always
revert to an earlier saved version.

* If you load a very large file (like an hour long), especially if it is
importing from a format like mp3, it will have a tendency to be much
more prone to crashing and even freezing.  I find that simply saving the
project immediately after loading a large file helps things, perhaps
because it frees some of the data in memory when it writes the project
to the disk.  Still, with very large files, there will sometimes be a
considerable delay before it will let you play an arbitrary location in
the sound file.  I find that selecting a small chunk of the file to play
helps things, probably because it has less to buffer into memory before
playing the small chunk.