I do keep a spare set of clothes in the car, but not in my jump kit. Only clothes there are an extra pair of socks, a wool cap, and some work gloves. Otherwise, my jump kit aims to supply me with the basics of food, shelter, water, first aid, navigation, rescue, and heat (even apart from fire).
Food can vary but include trail mix, MRE, powdered soup. Shelter is a tarp and paracord, yes there is a knife. Water is present as a half-liter bottle of water and also a filter straw. In the first aid kit is also some iodine (real iodine, not tabs). There are also a few quart sized high quality bags for holding water. There is a mirror and amateur radio (rescue), compass (navigation) and maps, and a few chemical heat packs. Other miscelaneous items include flash light, paper and pen, extra batteries for light and radio, hi-gain antenna, mylar space blankets (2), a few garbage bags, fire starters, small binoculars, and I know I must be missing something...
There are lots of good kits and ideas at this site:
http://www.sonic.net/~quine/jumpkits.html
Have fun...