Riding in the winter can be a challenge no matter how well prepared you may be. If riding in icy or snowy conditions I would recommend that you should already be confident on a bike in slippery conditions. If you don't know what to do when a bike slides out you should probably get some practice in more benign conditions.
That being said, there are a few simple techniques you can use to keep you out of trouble when riding in bad conditions:
When riding on icy back roads you will often find that the surface has been churned up and re-frozen. The general rule is to stay loose on the bike. Most of the time if you have reasonable momentum the bike will find its way out of trouble without too much input from the rider.
The exception to this is riding through the ruts left by vehicles (tractors and farm machinery seem particularly bad for creating ruts). You have two choices. You try to ride the rut. If it's wide enough and smooth enough this can be a good thing to do. Be careful that you don't hit the side of the rut as the bike will try to climb out and may throw you out of control. The other option is to cross the rut. I approach this in the same way as crossing tramlines or wet tree roots. Cross the rut as close to 90 degrees as possible. While crossing the rut use only minimal input to the bike, no brakes, no steering, no power, stay loose.
Once the snow gets above about 20cm deep my bike starts to struggle. I can find grip, but the weight of the snow being moved aside becomes too much for forward progress. If you have ultra-wide tyres you can float across the deeper stuff without the same problems.
When I have no choice but to get through the deep stuff I try to carry as much momentum as possible. I pick a low gear, get a good spin going and try to maintain speed throughout the deep snow. Shifting your weight back a little bit also helps. It will give you a bit more traction on the rear wheel and allow the front to float slightly.
Controlling a slide
Even with the best prepared bike you can find yourself in a slide. The nemesis for my bike seems to be unconsolidated loose slush - everything else is fine. Once you're in a slide you have to decide quickly if it's recoverable. Most of the time you can get control back by shifting your weight to counteract the slide and possibly dropping a foot off the pedal (flat pedals are handy here!). Again, stay loose and go with the slide.
If the slide is terminal you need to decide how to crash. If possible lay the bike down flat and push it away from you. If you're heading for a ditch or hedgerow again try to get the bike away from you and if possible aim for something soft!
When road riding in winter you should be acutely aware that most other road users won't have nearly as much grip as you have with your spiked tyres. It's all too easy for a car to slide wide on a hill or corner. My general rule is if I see a car on a piece of road that could possible cause problems I will stop, get off the bike and get off the road. It's not worth the risk of getting squished by a sliding car.