I have experience using programs like Izotope RX, Audition, and Audacity to remove background noise. Audionamix created an audio plugin for using in post production called IDC (Instant Dialogue Cleaner) that does magic for outdoor recordings and for recordings with machines running in the background (cars, fridges). Audionamix also offers some cool tools if you have music in the background that you need to remove (TRAX Pro).