There are a number of things that can cause this.
Go to system preferences>energy saver, make sure wake for networks is not checked, check print/fax that there is no jobs on hold ,if there are delete them. Check in accounts> login items, get rid of those, all these are in system prefs.
Bluetooth devices can wake it.
Here is more of the list, I just gave you the common culprits.
hope this helps.