Rainbow Patches and Puddles
The road surface will be the slickest and most dangerous just after the rain has begun. During this time the rain will cause the oil buildup in the pavement to rise to the surface, causing the road surface to essentially become a giant oil slick.
Keep an eye out for little rainbow-edged patches on the street. This is an indication of an oil patch. Never brake or corner in the center of the roadway at intersections, as this is where autos leave the majority of their drippings. Make an effort to notice metal surfaces such as manhole covers or steel-grid bridge decks, painted traffic markings, or wet leaves, as they all become very slick when wet.
As fun as it may be to bash through puddles, avoid them. More than one cyclist has broken a wheel on a submerged pothole—or submarined into a construction pit that appeared to be a harmless stretch of standing water.
Watch Those Brake Pads
A mixture of road grit and water turns out to be the fastest method to erode rubber brake pads. Pads that last a full season in the dry will run metal to metal on the rims in a month of wet-weather training rides. Also, most rims require a full revolution before the brake pads squeegee the water from the braking surface and begin stopping. Plan ahead and brake early.
Watch the Corners
Cornering in the rain can be tricky and dangerous. Shift as much of your weight on the outside pedal as possible. Use body English to keep the bike more upright when cornering. Lean your body more than the bike. By doing this, you will be able to corner with a reasonable amount of speed, as the body will tend to remain balanced over the bike when the tires slide over painted lines and unseen oil patches.
Fenders are not foolishness. If you plan on riding for any length of time, or with others, install fenders on both wheels. It will quadruple your comfort factor and keep your vision crystal clear. It's one of the unspoken rules that on rainy-day group rides you should never show up without fenders.
Drafting through a thin stream of grimy water gets old in about 10 meters. Several bike companies sell clip-on fenders that mount to the seatpost and downtube to fit bikes without threaded eyelets.