A number of users have reported an issue in which Mac OS X 10.6.x (Snow Leopard) is slow to go to sleep. In these cases, the system will stay awake for 30 seconds to several minutes before entering sleep mode.
Check printer queue. This problem may be caused by items stuck in the print queue. Go to http://127.0.0.1:631/jobs/, which will show all current print jobs. Cancel any stuck items, then re-attempt sleep.
You should also go to Print & Fax in System Preferences and remove any printers that you no longer use.
Disconnect external devices. Errant external devices, particularly USB drives and other peripherals, as well as iPods, iPhones and iPads, can cause a delay in sleep. Disconnect any peripherals then re-attempt sleep.
Check for hung processes. Launch the Activity Monitor (click the Spotlight icon in the upper right corner of the screen and type Activity Monitor), then sort the items by the CPU tab. Quit any items not in use that are using significant processor time.