Some strategies you could consider:
- Use a hand-pump garden sprayer to hose down your bike with warm water (store inside afterwards). $20 at your hardware store
- Replace all bolts with stainless steel bolts. Grease threads. Only use stainless steel cables.
- Consider a more rust resistant chain
- Protect inside of steel frame against rust (Frame saver, Boeshield T-9, boiled linseed oil)
- Regularly keep your drive train free of gunk and sand
- Touch up on all scratches and paint chips on your steel frame BEFORE rust starts
- Aim to replace as many plain steel parts with stainless steel or aluminum versions as reasonable.
- You can use something like thick grease or Vaseline to cover up moving plain steel parts, such as derailleur joints, etc (not chain!). Needs cleaning and replenishing once a season
- Use nosed housing ends and other covers to reduce water ingress into cable housing (esp. those that are pointing upwards and where water can collect in a housing)
- Full-coverage fenders with long mudflaps
Edit: in agreement with Jim (below): the best bikes go in winter storage inside. With good care (and using some of the tips above) you can ride a nice bike through winter and not wreck it completely. But wear and tear is higher in winter, so you are going to replace components a little earlier. I wouldn't want to see this with my $400 tandem bike hubs. With my standard disc hubs of my decent-quality hardtail MTB (winter commuter), no problem!
If you don't want the additional maintenance and care, beaters are your best bet.