Been thinking around this one myself.
For the smaller gigs with not a huge amount of cables, I use cable ties to keep each cable seperate and a rucksack to haul them around. The extra rucksack pockets are handy for various connectors etc. If you get the larger rucksack, there will be enough space for mics in boxes
For the slightly larger gigs, a Stanley mobile work centre has two compartments to seperate 240v power leads from other stuff. XLR cables can go around a drum, such as a small garden hose reel. This solution keeps things relatively seperate without the hassle of carrying around too many boxes, which take up space in the car