Track all expenses including office supplies, postage, consulting transportation, entertainment, printing, advertising or promotional materials, education, legal services, accounting, taxes, equipment, maintenance-service, maintenance-parts.
I track parts and service separately so I’ll know when it makes sense to buy new equipment rather than fix the old. Research materials (including trade journals, newspapers and business publications), miscellaneous, interest and insurance are also categories.
There are industry-specific expenses but if you’re not sure which expenses are tax deductible, be sure to consult your CPA. If you discipline yourself to enter your receipts every Monday morning, it’s never an overwhelming job.