One thing I've learned is that downtime kills the bottom line, messes up your schedule and can ruin your relationships with your customers. Have a backup plan for as many things as you can. What if your truck breaks down, your main mower, blower, trimmer. You should have backups or some sort of contingency plan for all of the above because sooner or later it will happen.
Learn how to fix stuff yourself and bring tools and supplies with you at all times. Having some bailing wire, duct tape, expoxy (liquid and putty), a portable air pump, electrical tape and connectors and some common-sized nuts and bolts in stock can save you a run to the hardware store and keep you on schedule. And don't forget the fire extinguisher and first aid kit!!! I also bring a couple of scrap 2 x 4s, 2 x 6's and 4 x 4s with me in case I have to block something up or to support one corner of my trailer gate if I can park it on level ground. I also bring spare lubricants/fluids for everything I run as well as a spare belt for every belt on all of my equipment. I carry at least one jack stand at all times too.