Great-looking rain :
Model a single raindrop (FFD modifier applied to a sphere works well). For
best effect it should be very long and thin, tapering to a sharp point at
the top - about 10 times as long as it is wide. The faster the rain is
falling, the more elongated it should be. Use this raindrop as instanced
geometry for your particles. (Incidentally, have the rain falling at an
angle - rain hardly ever falls completely vertically).
Particle Motion speed Variation = about 35%
Particle Size Variation = about 50%
Object Motion Blur applied to particles.
Duration (frames): 1
Duration Subdivisions: 10
(You can lower the latter two for faster rendering time at reduced quality)
Shading: Phong or Blinn
Ambient colour: Black
Diffuse colour: Aqua (RGB 130,200,200 or similar)
Specular colour: White
Specular Level: 80
Opacity Falloff: In, 100, Additive
Create a bright Omni light somewhere in the scene. Exclude it from lighting
everything but the rain itself. The idea is to use this light as a
'catchlight' to put highlights on the raindrops. This helps give it a 'wet'
You also need to make your whole scene look shiny and wet eg. if you have a
brick wall somewhere, put a glossiness map on it.
I think the secret of creating good rain is to spend at least as much time
showing its interaction with the environment - puddle splashes/ripples,
water dripping from objects, trickling down windows, etc.