Hi Aerbabe, The simplest solution, in all but the very coldest weather, is to wear extra layers underneath your flying suit. The only things you then need to worry about are those that Genghis has already pointed out; gloves and something around your neck.
I fly Tiger Moths (up to 14 flights a day) and I find an Irvin too cumbersome, but others swear by them. We have to wear helmets (company rule) so my uncarpeted head doesn't normally suffer. If its raining, I put an old waxed jacket on over the top of my flying suit. I find my jaw gets the coldest, with the result that on a really cold day my speech gradually declines until it sounds as if I've had an injection at the dentist.
Tricks:
1. Tuck the first pair of trousers into your socks - stops drafts.
2. Wrap newspapers round your lower legs under first pair of trousers. (only needed on very coldest days)
3. Don't go into a warm room during the day until your flying is completed.
4. Be happy in your work.