I started off wearing trainers, tracksuit bottoms, t-shirt, jumper and hi-viz jacket (well it was October). I had practically melted by the end of my commute.
I now wear a cycling specific short sleeved top or a long sleeved running top depending how chilly it is (you soon warm up, the first mile or so in short sleeves is quite cold but after that it's fine). I now much prefer wearing shorts. I have a cycling specific pair (Endura Humvee 3/4), a few pairs of running shorts and some
Regatta shorts from
Millets. I find the shorts from Millets the best with padded underwear (I really like the Endura CoolMax boxers). I like the Millets shorts so much I've just gone and bought two more pairs (£9.99 in the sale). I also have a few pairs of Ronhill Bikesters which are really comfy and warm in the colder months.
When it rains, I used to wear full waterproofs, but now I tend to just wear the jacket at this time of year as it's still warm. In proper winter weather I'll probably wear the whole hog - waterproof trousers, jacket, skull cap, gloves and overshoes. You do get hot but I much rather warm sweat than icey rain any day (and hail stones really hurt when they hit your poor uncovered little legsies).
I am also an advocate of going clipless, so much better. In fact I went out for a quick once round the block on my hybrid last night (to test out my camera on the helmet) and just stuck my trainers on (I have dual sided pedals on the hybrid), and even though it wasn't wet, my feet did slip a few times trying to pedal up the hill.