By being in the proper category you can get the allowance. If you are a sole proprietor, you are usually OK to get the deduction, as long as you follow the rules for business miles driven etc.
You will need to use a Mileage Log of some sort to show the IRS what miles you drove and where you went etc. Download the free Mileage Log.
If your vehicle is paid for, has a low cost of operation and gets a good MPG, then it might well be that this is the most lucrative way to deduct.
The law usually kicks in for the year on January 1 of the year and covers cars, vans, pickups and panel trucks.
In the year 2011, and a very few other years, we were showed a change mid year at the June 30, July 1 point. It was an increase for the second half of the year due to the elevated cost of fuel etc.
\This change and yearly changes are figured due to a study by the contractor Runzheimer International, working for the IRS.
There is also the Actual Expense method. That method is described more thoroughly in the web page linked in the previous sentence. If you are uncertain about which is the best method, then do a little math and figure out the numbers for each for the year. There will be your answer as to which will pay off best.