I keep a spreadsheet with entries based on the federal tax form "schedule E" (Rents received, Royalties received, Advertising, Auto and travel, Cleaning and maintenance, Commissions, Insurance, Legal and other professional fees, Management fees, Mortgage interest paid to banks, etc., Other interest, Repairs, Supplies, Taxes, Utilities, Depreciation expense or depletion, Other) and other details such as original purchase price, cash in, cash out, estimated property value, original mortgage amount, outstanding mortgage and any historical mortgage details if refinanced.