I used to be notoriously bad at keeping track of my sales/expenses at comics conventions. In the past, it's been clumsy lists or even just hatch marks on paper. During a slow day at a recent Portland show, I decided to put my iPhone and Google Drive to use. I'm not a speadsheet expert, but I mangled together something that I think works a lot better than hash marks on a post-it.
Using multiple tabs inside a Google Sheet, I can keep running tallies on my expenses and my sales. The first tab, labelled "Report" keeps track of the totals of the other two, automatically subtracting "Expenses" from "Sales". If all goes well, the "Profit" cell amount is a positive number.
One of the advantages of working in Google Drive is that I can immediately open the file in my desktop's browser and see the data. Even though it is really handy to input data on my phone, it's a bit easier to edit formulas and format the spreadsheet on a computer with a keyboard.