I live in Canada. I've dealt with breakdowns in remote areas - mostly aircraft but I apply this to bikes, now that I've given up the car.

First, I dress as if I were going to have to walk from whereever the "breakdown" occurred to civilization.

You can NOT carry enough parts and tools to fix everything on the bike that might break. You'd need a MIG welder, spare rims, hubs, bars. It's just not practical.

Remember this: TRIAGE.

Can the problem be fixed easily? Let's say the cable slipped out of the retaining nut on your derailleur. OK - a 4mm allen wrench to tighten it up. Easy. Even if it isn't perfect, you're limping away.

Your pedal or bottom bracket just siezed: don't touch it.
Lower the saddle all the way and PUSH with your feet. You can still roll along at 5mph using the bike as a scooter.

Forget servicing a freewheel. When it's -42, gusting 50mph and blowing snow, you'll likely do more harm than good fumbling with the guts of a bottom bracket, hub or headset.

In the time it takes to work on something like a hub, you're going to get hypothermic in the winter. If you really want to prepare for the worst, bring an emergency tent along you can service the bike in. That's overkill to me. I'll just limp.

TRIAGE TRIAGE TRIAGE. "Can I fix it here? Is it worth fixing it? Can I get away without fixing it?"

Tools: I used to carry a full tool kit for anything on the bike that wouldn't need "spare parts." (eg: tightening any bolt or nut, spoke, fixing flats etc.) I now carry a Topeak Alien. It has everything that was in my seperate tool kit including a chain tool.

Once the bike is at least limping away, pick the shortest route to civilization - even if it's in the wrong direction. Go there, make a phone call, get home or to some place warm. Survival is the key, not making a meeting on time.

I hope that didn't sound too heavy but I've seen people die when they didn't prioritize effectively during a crisis.