First off, make sure to have updated to at least 10.2.4 if not 10.2.5. Energy Saver has been much improved since 10.2.4. Use the combo updater.
Secondly, if you have a non-A/C powered device in the USB port nearest the modem that can cause problems with sleep. Also, you should put the mouse in the right side USB port of the keyboard and not the left.
Third party utilities that run in the background that startup usually from Apple menu -> System Preferences -> Login items can affect whether or not sleep works.
Make sure Screen Effects is not set to go off automatically if Energy Saver is doing the same thing. Check your Apple menu -> System Preferences for both. Also, don't let the machine go on Energy Saver or be off overnight unless you periodically run Macjanitor.
Wake up the machine from sleep only after giving it at least 15 seconds to be asleep. Use the mouse button and nothing else to attempt to wake it up.
Repair permissions and run a file system check (fsck) to make sure the operating system is sitting on solid ground.
Check your software update and Quicktime settings update to make sure those aren't on automatic, and get Info on your hard drive to make sure it isn't indexing while the computer is asleep.
Turn off any dialup internet connections before putting the computer to sleep.
Run the hardware test CD to make sure there is nothing wrong with your computer's hard drive.