I've found that often times it's difficult to crawl around under houses or in attics with tool bags or aprons on... it's also kind of hard to stick stuff in your pockets (tool handles get hung up in tight spots, cable staples impale you, hard to fumble around for stuff..)
We came up with a solution by accident.. went to the grocery store for lunch one day, grabbed one of those plastic hand baskets... took it through the self-checkout stand and walked right out the door without even noticing that I still had the basket. Whoops.
Anyway, the next day I had to spend some time under a house... the basket was still in the van... so I piled all my crap into it. Tools, materials, wire, everything. I tied a short piece of rope to it so now I can just drag it around with me. Works out pretty slick.