On those hot summer days ALWAYS ALWAYS ALWAYS keep iced water for yourself and the crew to drink. I also have an ice chest with Gatorade and canned sodas on the truck too after a job is done, gives us a quick pick-me-up in between rounds as we ride to the next job. Ain't nothing like a trip to the ER for heat stroke to learn this lesson the hard way.
Water hose, even a short 15 footer, to hose off the equipment after a dirty job, especially if it's rental equipment(Home Depot nailed me for that one when I returned a roto-tiller). It helps also to cool off too when the temps hit 100 or better. soak your head and even your feet, I know it sounds silly, but it really does work, pull off your shoes and socks and water down your feet if you feel hot, the veins in your feet will carry the heat and cool you off. (A towel stuffed under the front seat isn't a bad idea either if you try this).
I always take my guys out for lunch at an indoor establishment to help them cool off as they eat. Not only is it a nice treat for your crew, it's tax deductible too.
Keep receipts for EVERYTHING. I write off ice, fuel, supplies, equipment. A good CPA can figure out where to put it to good use and reduce your tax burden.
Write your mileage on the gas receipt. Calculate mileage used versus gallons of fuel burnt at least once a week, it'll give you a good indicator when you need a tuneup, even if your rig is running decent.
If you use a spool trimmer, duct tape a spare spool to the shaft, saves time going back to the truck. And invest the money and time to keep lots of fresh wound spools and sharp mower blades and edger blades on the truck at all times.
Not only is wasp spray a good idea to keep on hand, but so is fire ant granules. I do it for my regular customers as a freebie, not only do they appreciate it but it saves you the hassle of having to deal with them on a later date.
I learned this one the hard way, don't use a belt loop for your keys, if they fall off while your mowing not only are you stuck like chuck, but then you need to call a locksmith too.
For you newbies in the biz who do landscaping, go to the different nurseries and get some catalogs and/or price guides. Sure does help make bidding jobs easier if you don't have prices memorized. Always bid from the most expensive catalog just in case the cheaper supplier is out of stock and/or the customer is looking over your shoulder. Always check to see if the supplier has that item in stock, or that they still carry that item before you commit anything to paper. an illustrated handbook for your region isn't a bad item to bring with you when bidding jobs either. Here in Texas I rely on Neil Sperry's Guide to Texas Gardening as a bidding tool. You and the customer can browse the book together and get ideas, see if the particular plant will be suitable for your region and even help you to diagnose potential problems.
When bidding landscaping jobs, I always insist on a soil test if the customer wants a warranty on the plants. My excuse is you wouldn't build a house without a blueprint, why would a landscaper put in plants without an understanding of the soil conditions, nutritional surpluses/deficits. It makes you look professional and educated, gives you an idea of what you are dealing with and what you need to provide the plants with for optimal conditions to grow and thrive.