An emergency car kit is one of those things you never imagine having to use, but you'll sleep so much better if you know you have it. And more importantly, you'll actually improve your chances of a good outcome if you actually ever need to use it.
If you're going to be travelling away from the city and ready help, consider putting some or all of you emergency kit contents iinside the car with you - that way if you get rear-ended and can't open your trunk you can still access what you need.
Emergency Car Kit Contents
First aid kit
Non perishable food (i.e. energy bars, fruit bars, etc.)
Water (in containers that won't crack or break if the water freezes). Replace the water every 6 months at least (pick a time that's easy to remember such as when you change the batteries in your smoke detectors).
Extra clothing that you can change into if what you're wearing gets wet (i.e. socks, mitts, hats, shoes)
Something to cut a seatbelt
Candles, matches, and a deep can
Flashlight (preferably windup)
Radio (preferably windup)
Whistle (to attract attention)
Sand, salt or cat litter (non-clumping)
Extra antifreeze and windshield washer fluid
Small shovel, scraper and snowbrush
Warning light or cones, road flares