Get shelves and things. These guys do custom design work, and I believe they're just down the road from you.
Weight is your biggest enemy, even more than size. Anything you can do to reduce weight pays off in gas usage. That means taking equipment out of boxes if you can store it safely, and paying close attention to your stock usage so you can predict what you need to carry and readjust periodically instead of carrying everything around on the off-chance like a trunkslammer. Don't drive around with a thousand little spools of scrap wire. Get rid of clutter- take an hour a week to tidy up and half a day a month doing a top-to-bottom deep clean.
Make sure you have a maintenance schedule so you don't go too long between oil changes and tune ups.
Carry cleaning equipment- the lightest motel-quality vacuum cleaner you can find, a broom, a dustpan, garbage bags, cleaning spray, and paper towels. Cleaning up after yourself, whether you do commercial work or residential, is the mark of a professional.
Charged spare batteries for any battery powered tools or equipment you have, including walkie talkies and flashlights.
Do you have service agreements in place? If you do, see what equipment you've installed in the past, and try to figure out what you can carry that will be compatible and comparable with what you have in place. No more than two spare cameras, one indoor and one outdoor. Weight is the enemy!
Obviously, you can never have enough test equipment. Remember, spare batteries for everything.
Screws in every size and configuration. Nothing sucks as much as when you have to run down to Home Depot to get one screw or anchor to finish a job. Carry a third of a box at a time to save weight, and refill at the end of the day whenever you use anything.
Using CAT6 only and saving gas is cheaper than driving around with a box of CAT5e and a box of CAT6.
Only carry 1000' spools if you know you're heading out on a job. Otherwise, carry 500' spools.
Carry the lightest power inverter you can find.
Inventory control is important. Carry a clipboard with your normal loadout on it, write down what you took so you know what has to be replaced. This article about how to do "just-in-time" inventory for ambulances has some great tips.