With mudguards sorted, the next thing is to protect yourself from is the rain from above. The key here is to choose waterproofs with the right balance of waterproofing and breathability. Heavy duty waterproofs may keep you dry when you're standing still but you'll soon boil inside them even when cycling at a moderate pace.
Therefore, lightweight, highly breathable jackets and overtrousers are key. Often it's best to sacrifice ultimate waterproofing for breathability, especially if you intend to wear the same clothes on the bike and in the workplace.
If you're cycling in your rain gear, the key to keeping dry and cool is to take it slow. Even with breathable gear, you'll get hot quickly, so moderate your pace and adopt a ‘faster than walking' philosophy, resisting the urge to hit the hills hard or sprint away from the traffic lights.
If you've got changing facilities at your workplace and the time to do it, taking a change of clothes into work can often be the best bet. This way, it doesn't really matter if you get wet on the way to work. You can ride as fast and hard as you want, safe in the knowledge that you can get out of your wet things when you arrive.
Many cycle commuters choose to drive in on Monday, taking a week's worth of clothes with them, meaning that they don't need to lug clothes back and forth every day.