When driving in the rain, maintain proper distance, it takes longer to slow down in wet weather. Watch out for brake lights of the car in front. Do not follow large trucks and busses too closely, take extra caution when passing them, as the water sprayed by their tires may reduce your visibility.
A few maintenance of the vehicle before driving in the rain:
Lights: make sure your headlights and brakes lights are working, so you could see better of what’s in front and also give signals to whoever is behind you.
Tires: tires with less tread have significantly less traction in wet road conditions.
Wipers: replace them if needed. Also check your windshield fluids, use them to clean off the windshield before you drive in the morning, as light ice may have accumulated.
Foggy Windows: high humidity can quickly cause windows to mist up inside the car. Turn up the heat and direct the airflow to your defrosters with air conditioning on. In a car without an AC, you may need to open your side windows to get the air moving.
Last but not least, do not splash the pedestrians.
For a more comprehensive article of tips for driving in rain, read the full article at Smart Motorist.