start computer holding shift key...continue holding shift key while the gear spins until the blue screen appears...this takes longer than normal start up...when blue screen appears let go of shift key...once system boots up restart as normal...you'll be amazed at the performance benefits...
also use the disk utitlity once a week...
Applications>Utilities>open Disk Utility
select hard disk usually called Macintosh HD...
Disk Utility window appears...select First Aid tab...click Repair Disk Permissions...after it says permissions repair complete you may quit...
good idea to run these once a week at the same time ...