Flew through the rain once, but never got any explicit training, so here's my research on the subject...
- Flying in the rain is perfectly safe, just make sure you have VFR conditions - visibility, cloud base is sufficently high to fly above MOCA
- Losing altitude - pitch for Vy. (expect downdraft: rain cools the air and cold air falls...)
- Open up the side window for a good reference to the ground
- Be on top of weather conditions at all time, call flight watch
- Make sure your lights are on
If it's getting bad enough on the windscreen that you can't see well in front, make shallow S-turns to get a good look ahead out the side windows. - You can hydroplane on the runway same as the roadway. Wet surfaces also take a longer braking distance
- Be sure to have heat or good airflow to avoid fogged windows.
- When you come in to land you seem to be lower to the ground than you really are.
- If it has just begun to rain, all oil/dirt will rise to the surface making it very easy to slide off
- Don't hit the brakes hard.
- Use carb heat, not only in the event of icing, but as an alternate air source, to prevent the injestion of water into the engine.
