As the amount of memory in Mac notebooks has grown, it takes longer to put them to sleep because OS X defaults to writing the entire contents of memory to disk before going to sleep. APC magazine Web Editor Dan Warne offers up two ideas for solving the slow-to-sleep problem:
You can disable the safe sleep mode by opening the terminal and typing:
sudo pmset -a hibernatemode 0
And to get safe sleep mode back, change the "0" to a "3".
Another option is to use freeware app Smartsleep, a preference pane which gives you the option of letting your notebook just sleep when the battery is close to fully charged, or if the battery level drops below a certain point it will switch to sleep and hibernate. As Dan says, if you usually work with your Macbook plugged into a powerpoint, this is ideal.