Always keep a stash of stuff to eat in one compartment along with a case of bottled water, some jobs turn into marathons and always happen where there's no stores or places to eat. Electric cooler is good, if you have a welder/generator a mini microwave is priceless. Probably not the kind of advice you're looking for, but my attitude goes South real quick when a four hour job turns into twenty and my stomach thinks my throat's been cut. Change of clothes doesn't hurt either. I have taken oil baths that soaked me to the skin even wearing coveralls. Diesel is the worst.
If you're working jobs where there's nobody around (I did constantly when I was in the field) develop a habit of thinking to yourself before every procedure "Now if I do this, what could possibly go wrong and could I be injured or trapped in some way?" We've all heard the stories of techs loosening hoses and ending up crushed or suffocated by whatever came down. Same goes for driving pins out, removing belly pan bolts, blocking machines up, you get the idea. Safety is always top priority, but in the field even more so and it's more challenging.
Good idea to keep a sidearm handy in the truck, YMMV, but you just never know when some form of two or four legged animal might need talking to in a loud voice. Some may find that distasteful, but like a fire extinguisher I'd rather have it handy and never need it than desperately need one and not have it. Oh yeah: CHECK YOUR FIRE EXTINGUISHERS AND FIRST AID KITS at the start of every day. The one time I really needed an extinguisher somebody had used mine and politely put it back in it's holder, empty. Wasn't much use to me.
Keep your sense of humor, you will undoubtedly be dealing with all manner of people out there in all kinds of moods, be professional, polite, a calming influence, and the kind of driver that assumes he has to keep everybody else around him safe because they're not smart enough to do it for themselves.
Lastly, have fun! It's a completely different world from working in a shop.