Your service truck is your storefront and your workshop, and a well-organized truck makes you faster and more professional. The basics of what to stock: every common wire gauge in short runs, a selection of common breakers (for the panel brands you commonly encounter), boxes (metal and plastic), connectors, staples, assorted hardware, a selection of outlets and switches, covers, tape, wire nuts of every size, and your diagnostic tools.
Organization systems: Sortimo, Weatherguard, and Knaack make professional van racking systems. A properly racked service van has a place for everything. The upfront investment ($2,000-5,000 for a quality racking system) pays back quickly in time savings and professional impression.
Material replenishment: after every job, note what you used. Weekly restocking run to your electrical supply house. Running out of common material on a job and having to make a supply run kills your productivity and profitability. On T&M jobs, charge for material trips as time. On flat rate jobs, material runs eat your margin.