I try to plan for Murphy as much as possible. Once of every spare bulb and fuse as well as spare fan belt. Long trips I take box with a change of oil, transmsiion fluid and premixed, aintfreeze as well as a tube of form a gasket. Should be enough to reseal something such as an oil pan temporarily, refill the oil and get to a store for a real gasket. Swapped compact spare for full size and do 5 tires rotation pattern to keep spare in use so I don't end up with a flat spare when needed. Small 12v compressor and tire plug kit in case of multiple failure.
I've started trying to keep all my gear/tools/supplies in their own bags now. Have wrench rool stuffed in the jack storage and another tool kit behind a rear seat. Another "surival kit" is behind the other rear seat, small first aid kit is under the drivers seat. Try to keep the same habit at home, computer, cd burner, backup cd's, tools, kits, etc each have their own storage bag. Depending on the situation and what I have to carry with, be a truck or sneaker, I can grab the appropriate bag with the gear/kit I need and go. Helps to keep things organized as well since everything has a place.