I use Quicken and keep my business and personal checkbook updated on that on a monthly basis. It's easy to do and at the end of the year you can print out everything you've spent and what you've deposited (provided everythings on the books, yeah right) and it gives you a good idea of where you are financially. It's easy to do and easier for your accountant at the end of the year. Save all cash receipts.
If possible use a credit card for your gas and write 1 check at the end of the month and keep the cash in your pocket.