How do you tie a knot in a tie like the models at department stores
always have - large, perfectly triangular, with a crease perfectly
down the middle?

You definately want the Windsor, it's very precise and if done right,
perfectly symetrical.  The half Windsor is smaller than the
full/double Windsor.  I think the half Windsor is easier to knot than
the full. Here's some tutorials with pictures:

How to tie a necktie
http://www.mckinnonsc.vic.edu.au/school/ties/ties.htm
This is a great link!  ..."Which knot for my tie? Use the Double
Windsor for thin materials such as silk, and with wider ties.
Hand-knit or woollen ties are too thick for the Double Windsor; use a
Four-in-hand when you need a smaller knot..."

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Windsor Knot (also called "Double Windsor")
http://www.tie-a-tie.net/windsor.html
Great graphics.

Half Windsor Knot
http://www.tie-a-tie.net/halfwindsor.html

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"How To Tie A Tie"
http://www.scoutdb.org/h2tat/

How to Tie a Tie
http://www.ehow.com/how_2970_tie-tie.html

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Good luck!

~~Cynthia

PS - You may wonder how I know which knot is easier... I actually wore
a tie every day for years, in Boarding School!

I believe the measure of a man is his tie knot. Large triangular tie
knots with short wide ties that don't even go down to the navel seem
to be the vogue. Men I've known with tie knots such as these are
loudmouthed, have something to prove, yet prove to be of very little
substance indeed. Men with tiny tie knots have nothing to prove (or to
compensate for). Disregard them at your peril.

The best necktie knot is the "half windsor" which, despite its
inferior-sounding name, is a much better choice than a "full windsor."
 A full windsor is sometimes considered better (the half has a little
less symetrical), but it is almost impossible to do neatly with all
but the thinnest neckties because it is a much larger knot.

Here is a good instruction set for a half windsor:
http://www.tieguide.com/half-windsor.htm

It is especially good if you want to have room to get the center
crease just right.  A full windsor is too tight and leaves your tie
too short, and the simpler four-in-hand most people know just looks
horrible.  The half windsor can be tightened or loosened to your size
preference.