Section 179 deductions are not automatic. You must elect to take it when you file your tax return for the year. The deduction is taken on an item by item basis, so you do not have to use it on all eligible property bought during the year.
 
Steps:

First, keep complete records of the business equipment that you purchased or leased during the year.  Note where you acquired the equipment from, as well as the date it was acquired and date it was placed into service.

Second, when you are filling your tax return, fill out Part 1 of IRS form 4562 and attach it to your tax return.

Download IRS form 4562 for tax year 2012
Download instructions for IRS form 4562 for tax year 2011
 
Note that under Rev. Proc. 2008-54 you are now able to amend and elect a Section 179 deduction for previous tax years, if you did not elect for tax years beginning after 2007.