The following are step-by-step instructions on how to whistle using your fingers:
How to whistle loudly using two fingers
"I find the easiest two-fingered loud whistle to do is to use my index
finger and thumb of my right hand. Here's how to do it:
Loosely touch the tip of your bent index finger to the tip of your thumb.
Rest the bottom of your joined fingers (if you're using your right
hand, the thumb is to the right) on your lower teeth with the tips of
your loosely joined fingers pushing against the bottom part of your
tongue (which pushes back).
Your lips should be pressed down against your fingers. The joined
fingers make the "V" into which you blow (you should blow slightly
down).
It definitely makes a shrill sound that gets louder as you blow
harder. Of course, this method works best if your nails are short."
Whistling Lessons
http://members.rogers.com/l.hamilton/lessons.html
Fingered Whistle
"Tuck away your lips
First, your upper and lower lips must reach over to cover your teeth
and be tucked into your mouth. Only the outer edges of your lips are
visible, if at all.
Choose your finger combination
The role of fingers is to keep the lips in place over the teeth.
Experiment with the following combinations to discover which works
best for you, depending on the size of your fingers and mouth.
Regardless of your choice of fingers, their placement is the same:
each are placed roughly halfway between the corners and center of
lips, inserted to the first knuckle. (Again, this will vary depending
on the size of your fingers and mouth.)
Your options are:
a U-shape created with thumb and middle finger, or thumb and index
finger, of either hand.
right and left index fingers.
right and left middle fingers.
right and left pinkie fingers.
Now that your fingers are in place, be very clear on these two matters of form:
1.) Your fingernails should be angled inwards, towards the center of
the tongue, and not pointed straight in and towards the back of your
mouth; and 2.) your fingers should pull the lower lip fairly taut.
Draw back the tongue
Now comes the crucial part of the whistle.
The tongue must be drawn back so that its front tip almost touches the
bottom of the mouth a short distance behind the lower gums (about 1/2
inch/1 cm). This action also broadens and flattens the front edge of
the tongue, allowing it to cover a wider portion of the lower back
teeth.
The sound is produced by air flowing over a bevel, or a sharply angled
edge. In this case, the sound is created by the upper teeth and tongue
directing air onto the lower lip and teeth.
Blow
Steps 3 and 4 follow each other very closely, if not simultaneously.
Inhale deeply, and exhale over the top side of the tongue and lower
lip, and out of your mouth. Some extra downward and outward pressure
by the fingers onto the lips and teeth may be helpful. Experiment with
the position of the fingers, the draw of the tongue, the angle of the
jaw, and the strength of your exhalation. Adjusting with these will
bring success.
Start off with a fairly gentle blow. You'll produce a whistle of lower
volume at first, but you'll also have more breath to practice with if
you don't spend it all in the first three seconds. As you blow, adjust
your fingers, tongue and jaws to find the bevel's sweet spot. This is
the area of maximum efficiency, where the air is blown directly over
the sharpest part of the bevel. Once you locate the sweet spot, your
whistle will have a strong, clear tone, as opposed to a breathy,
low-volume sound.
Listen for these sounds: as you practice, your mouth will learn to
focus the air onto the bevel's sweet spot with increasing accuracy.
You'll probably hear the following: a breathy, low-volume tone that
suddenly, as you adjust your fingers, mouth, or jaw, will switch to a
clear, full, high-volume tone. Success! You're on the right
track--your task now is to reproduce the mouth and hand position that
led to the better whistle."
How To Whistle Loudly
http://www.natwilson.com/stuff/whistle.html#intro
http://bluebones.net/whistle/
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This site teaches you how to whistle using your hands:
Two-Handed Whistling
http://dolphin.upenn.edu/~thompson/whistle.html
(originally posted by Google Answers)
Satchel Paige's Guide to Longevity 
