An audit can occur at any time, so be prepared. Here are some guidelines:
Prove you have the licenses. If individual departments are allowed to make software purchases, make sure they submit copies of the licenses and purchase orders to a central location. If an employee installs software without a license, your company may be liable, even if the application was installed against corporate policy.
Keep tabs on your license count to help reduce costs. Instead of guessing how many licenses you need, buy exactly what you need or close to it (always buy a few extra licenses, just in case new machines are added).
Use software metering (tracking the frequency with which an individual uses a program) to determine if you really need a license for every user or if you can share. Make sure to uninstall unneeded software or bar inactive users from accessing the product, as most software licenses are based on per-seat installs, not simultaneous usage.
Of course, desktop-management suites let you see exactly what is installed and where. So, if you get an audit alert, you can reply in minutes with, "We have 4,830 copies of Windows 2000 installed, 4,293 copies of Word, and have the licenses for 5,000 each. Have a nice day."