Hi folks - first post on this forum, but I fell foul of the winter bike ride thing last year, so thought I would post what I learned.
I went out for a ride on a distance I should have had no problems with on a frosty day. After about 6-7 miles I was realising I wasn't happy, and fortunately managed to get the bike onto the grass verge before I actually blacked out and fell off! That was the point the mobile phone came out and the wife got called, and it spooked me for a while.
I worked out two reasons for it - the first was clothing, and the suggestions above are great - I went out and bought myself something called a "base layer" that made a massive difference. But I think the big one was food - I went out more or less without any breakfast, to make an early start. I think I simply ran out of sugar with the cold and a couple of hills.
Hope that's some use!