Factors affecting meringue crispness
I recommend watching Egg Files VII - Good Eats. It is an entire episode on meringue.
http://www.youtube.com/watch?v=TmIuFX3x_ik

Important factors affecting the result:


temperature
amount of sugar
how the sugar is incorporated 


Alton Brown explains how to make three types of meringue: french, italian, and, swiss.

Here are a tidbits of info from that episode:

French Merigue 


add pinch of salt to egg whites
whip egg whites, 
add sugar slowly until dissolved
add vanilla extract
add cornstarch and vinegar
bake, then turn off oven and wait 3 hours with oven door ajar
low temperature 250 degrees fahrenheit


The low temperature means the meringue dries out slowly. 
The outside becomes dry (crisp) first, and since the 
temperature is dropped before water inside can escape
the inside becomes soft and moist. 

The acid from the vinegar helps the structure of the foam (note: new eggs 
are slightly acidic, so use new eggs for meringue).

The corn starch prevents any liquids from seeping out.

Swiss


add pinch of salt to egg whites
dissolve sugar in egg whites
whip on medium while heating (Alton uses a hair dryer!) 
wait until the temperature hits 140 degrees fahrenheit
kill heat
whip on high
These meringues were poached 


These meringues becomes very gooey inside.
Note: Hot water can dissolve more sugar than cold water.
      Sugar keeps water in the final product.

Italian


whip egg whites with a litle salt and vanilla extract
make hot sugar syrup with sugar and corn syrup
slowly drizzle syrup into foam while mixing