I know owner-operators who only track the revenue on a load and how much the fuel costs will be. They run their business (or hobby I should say) like that and wonder why they can't get ahead. That's a classic example of a truck driver who buys a truck and continues to act as a truck driver. To succeed, you must be a business person first and a truck driver second.
I track all fuel purchases by writing odometer reading on receipt and then 
recording price, gallons, fuel mileage and state where purchased. All this is needed for IFTA.
Here's a list of what I track monthly on a spreadsheet: These are what I call KPI's or Key Point Indicators.
1. Total Miles - from home until I get back home
2. Revenue in Dollars
3. Revenue per Mile -from home until I get back home
4. Average Fuel Cost
5. Fuel Mileage
6. Fuel Costs per Mile -Fuel Cost per gallon divided by Fuel Mileage
7. Cost to Operate -I have a fixed number for the other expenses and I add my fuel cost per mile for that month to obtain this number
8. Profit - Revenue per mile that exceeds my cost to operate per mile
9. National Fuel Price Average -www.eia.gov
10. Remarks - anything to add context to the numbers; ie. No deadheading, out sick for a week, truck down for maintenance for 4 days, etc..
I've found that by tracking these KPI's monthly, I'm able to easily identify areas for improvement. I'm able to accurately determine how much profit I'll make on a certain load and be in a position to compete more aggressively with the competition. Start tracking your Key Point Indicators (KPI's) to make 2015 your best year in business ever.