Mark,
Dendritic growth is a corrosion mode generically refered to a electromigration.
Conductor will dissolve into a water film on your board.  These metal ions will
move under the influnce of the circuits applied voltage and "plate" onto the 
opposing conductor.  As more metal dissolves, moves and plates out, a metal
crystal begins to grow along the lines of potential, eventually causing a short
circuit.  
Electromigration needs three components to occur:
1.) A voltage difference
2.) Ionic contamination
3.) Moisture
Voltage differences can be controlled via design.  Keep grounds and the high
voltages seperated as much as possible.  Avoid layouts which concentrate 
electric fields, such as sharp corners, points, etc.  Use the lowest voltages
required for the application requirements.  Keep in mind that the closer your
conductors, the greater the electric field strength, the more chance of 
dendritic growth.
Ionic contamination can be controlled via process and materials selection.  
Keep you products clean.  Check ionic levels of incoming material.  Check your
handeling procedures (finger prints, flux residues, etc.).  
Moisture is often controlled via conformal coatings.  These should be considered
stop-gaps until proper design and process controls can be implimented.
IPC has test metheod, coupon designs and guidelines.  I don't know all the 
document numbers, if someone else does, please respond.