All I do is open a spreadsheet in Excel. First column has the date. Second column has my income, the next my expenses. It's all totaled at the bottom. Easy, fast, etc. Then I just put in a note in the next column as to what the money was for. I use a new line for each new transaction, whether that's income or expense.
At the end of the year, I know what my gross income was and my net income.
I keep all business receipts, but those numbers should already be on the spreadsheet if you were accounting properly through the year. The receipts are just for if you're audited.