1. Combat the Cold
Keep warm by moving around. Move your arms and legs one at a time on the bike, and stop more often to warm up and get moving! Even when stopped at red lights, use the pause to roll your wrists and arms.
Take advantage of the hottest part of your bike: the engine. When stopped, bring your hands close to it (without touching it, of course) to warm them. Stopped for a few minutes? Place your gloves on or near the engine, either tucked under the body, or placed on the cylinders if you have a flat-twin.
When warming up, bring your hands close to a heat source (a radiator, for example), but don't make full contact! Cold numbs the body: there is nothing worse than plunging cold hands or feet into hot water; this can cause burns or other serious problems. Instead, place them under warm water, and very gradually increase the temperature.
At the end of a long winter trip, it can be tempting to speed up or book it on the last leg to arrive faster. This can be a deadly mistake! During those last few miles, the driver is the most tired, the most careless, and therefore the most accident-prone.
2. Beware of Road Hazards
Increase the distance between you and the vehicle ahead. Not only might the ground be slippery, which increases the stopping distance, but if there is snow or ice on the road, dirty snow can accumulate in the wheel arches of cars and trucks. These can suddenly break off and fall directly in front of your tires...
Another wintry hazard is the film of frost and ice that can form on the bodies or tarpaulins of trucks, which can break off at high speeds or in windy conditions. This can send icicles into your face.
Beware of plows and salt trucks that project salt, sand, or snow to their sides or behind themselves. Give these vehicles extra room, and if you see one, slow down and keep as far to the right as possible.
Don't think that you can safely follow a salt truck. The salt it spreads on the road will only melt the snow or ice after about 20 minutes. Even worse, salting can create moisture on the road, sometimes even resulting in an extremely slippery film of grime, slush, and other materials.
4. Rain, Fog and Mist
Cold and dry weather doesn't present any particular problems. But don't forget that when the air is colder, it will take longer for your engine to reach its optimal temperature, and longer for your tires to achieve optimal adhesion.
In rainy weather, the best thing to do is to slow down. Also remember that you get cold five times faster when wet. Stay warm with watertight apparel! Otherwise, stop to dry off.
In foggy weather, the road is wet, and the same advice applies as for rainy conditions. But you also have to factor in reduced visibility for yourself and others. Slow down even more!
In addition, make sure that your low beams are on. If you can, turn on your hazards, or turn on fog lamps if your bike is equipped with them... High beams become useless; the intense light beam is reflected by water droplets suspended in the air, causing even worse visibility, especially at night. If possible, wear a reflective vest or LED armband.
If you wear glasses, don't make the mistake of raising the helmet shield.
Condensation can immediately collect on the lenses of your glasses, and you will quickly become almost blind. If the shield fogs up, partially open it, but do not open it completely. Frequently wipe the outside of the screen with a gloved finger, or with a squeegee if possible. If you plan to drive in fog, make use of anti-fog products or a double screen "pinlock" system.
5. Frost, Ice and Snow
If it rains and it freezes, you'll likely encounter ice. This increases the risk of your tires losing grip. Be especially careful in windy places (bridges, gorges), or in tunnels, shaded underbrush, north-facing slopes, etc.
If a sheet of ice is straight and not very long, you have a chance to end up back on the asphalt. Don't brake, look straight ahead, and do not tense up. But if it's a turn, it's over; you're down. And all roads eventually turn... Don't tempt fate.
If it snows, get ready to rock 'n roll... Wherever possible, if you know snow is on the way, avoid taking the road on two wheels. First, it's slippery, and visibility is particularly poor. In addition, the snow will stick on the helmet visor, quickly rendering you blind, so be sure to wipe every ten seconds to see.
Riding in fresh snow and powder is no more complicated than on a wet road. Try to follow in the tire tracks of vehicles ahead as best as you can, avoiding places where the snow is caked down, where it's compressed and often icy. In the snow, stay in second gear, with both feet out to catch small slides, without braking or accelerating. Keep on the clutch! Get the feel for the clutch slip point, and go easy on the gas.