I purchased a new helmet cover – a Louis Garneau H-Cover.
My previous cover was a one-size-fits-all variety from MEC. It was too large for my helmet, would move forward onto my face and flap around in the wind. The Louis Garneau cover is designed for their helmets (I wear a Quartz helmet) and is available in small, medium and large sizes like their helmets. The H-Cover fits perfectly over the helmet providing water and wind protection in bad weather. I find I am not just using the cover in wet weather. It blocks the wind so, in colder weather, my head stays warm without additional protection like a cap or touque. The cover also features reflective logos and and piping on the front, sides and back making you more visibile in low light conditions.
If you have never tried a helmet cover and ride in wet, cold weather, I would recommend you try one. I find I use the cover more than expected – not just in wet weather. It breaks the wind, keeps the head warm but, because it is also breathable, the head does not get overheated.