That's a real toughy actually - water is a funny beast.  It has a fairly unique property known as "surface tension" that causes it to react in strange and wonderous ways.

Ever wonder why people jumping off a high bridge die when they land even though they are hitting water?  Surface tension - hitting water at those speeds is like hitting concrete.  Same reason a modern aircraft carrier can stay afloat even though they weigh more than 35,000 tons (or roughly 7 million pounds).

At any rate, because water has this cohesive effect, it tends to react very differently to different types of surfaces.  Water on a horizontal surface will tend to bead and pool, but when presented with a vertical surface it will tend to stream or drip.

I would imagine a mathematical model of this would be highly complicated - certainly far above my meager math skills.  I imagine some incredibly brilliant individual with 6 or 7 advanced degrees in math and an intimate knowledge of the python programming language might be able to come up with a python program to do an accurate simulation based on the type of surface and amount of water.  I have enough problems balancing my checkbook and the closest I ever got to python was the local zoo.

Me personally I went the lazy guy road and just made a set of various brushes for photoshop and put it all in post production based on what I think looks good.  Not scientific and certainly not accurate but most people probably wouldn't notice the difference.

As far as the part where water would be dripping from a vertical surface (lets say from the fingers or the chin for example) my guess would be you'd probably need additional geometery of some sort to accurately simulate this in program like poser, a prop file of some sort would probably work, but again nothing I can think of to automate the process of adding them without some really high end math or script.

If you do come up with a solution I'd love to hear it, it's one of the tougher questions to crack to be certain.