1. Remove unused applications;
2. Close unnecessary notifications;
3. Repair disk permission
--a. open applications->utilities->disk utility
--b. select Mac OS disk
--c. verify disk permissions
--d. repair disk permissions
4. Repair Disk
--a. insert Mac OS CD
--b. restart the system
--c. open utilities->disk utility
--d. verify disk
--e. repair disk
these few steps have worked for me, it feels light and new again.