Having your system go to sleep when it is inactive is a great way save some money on the electric bill and prolong the life of your system. Unfortunately, there are a number of situations where your Mac will not sleep automatically and very little in the way of helpful diagnostics or documentation to guide you is available. Apple provides some basic information on reasons why sleep might not be working here. If you've read the article and still have problems getting your system to sleep, here are the most common things to look at:

1. Some applications such as iMovie seem to prevent sleep from ocurring automatically, try quitting all applications to see if this will allow the system to sleep.
2. Check the System Preferences->Energy Saver->"Put the computer to sleep when it is inactive for:" setting and ensure that the value is set to a value other than "Never." Also, try changing the setting to a low value (i.e. 5 minutes) and gradually increase to your desired value... if you have a periodically a running task(s) that is preventing sleep mode from being entered, this exercise might help you determine it.
3. Check System Preferences->Sharing for any enabled shared services and try disabling them. Any active shared system service (i.e. file or web) will prevent sleep.
4. Check log files for activity. Sometimes, it is possible for activity to be generated at a system level which will prevent the system going to sleep. The console.log, system.log, and ipfw.log are the first places to look.
5. Start unplugging USB devices one by one to see if there is a particular device that is waking the computer. This is also a good way to confirm what is indicated in the system logs.