Firstly, take the scientific approach by predicting the actual temperature is your best bet! Websites like yr.no and Accuweather are excellent at predicting actual temperature with windchill. Mobile apps like Dark Sky that also work offline are excellent as it may help you pick the best route to avoid showers.
Let's start with the worst weather combination -- wet + cold. If it's wet and cold for the whole ride, this is the most challenging. The priority should be a warm waterproof winter cycling jacket, waterproof / neoprene gloves and overshoes. Roubaix Bib Tightsare perfect for all Winter weather conditions. A peaked cap to keep your head warm which also preventing dirt from getting on to your glasses is good in both Winter and Summer. Winter boots are getting popular recently so they might be a good investment.
Showery + cold conditions. One should dress the same as the wet and cold as the roads can remain wet for a long period of time. First and foremost, dress for the cold and have a waterproof cycling jacket that you can fold away into your back pocket when it dries and this should stop you from overheating. Some waterproof type jackets are not expensive as they are not breathable. The breathable membranes produced with materials like Gortex and eVent are good but more expensive.
Dry + cold One needs to dress for the cold with breathable membranes but also be aware of the windchill factor. It's important to put extra emphasis on the extremities, therefore include an under helmet cap, warm gloves and shoe covers--you can remove them if the day warms up. So, a good quality winter cycling jacket with three pockets on the back is ideal for carrying what you don't need to wear.
In wet + humid conditions dress for the rain and wear less layers. There is a lot of choice at the moment when it comes to a lightweight waterproof breathable membranes that don't heat up. These have become very popular in recent years. Velotec have our Race Dry jacket and new waterproof gilet coming soon. If you cycle competitively, you can also use one during the early season races being careful not to damage the membrane with safety pins. Remember always wear a mesh type cycling undervest. These are a cycling essential in my view in both the Summer and Winter.
Visibility is also a priority, so if you wear dark clothing, make sure you have good lights on your bike. Be safe out there.