1. Run your Disk Utility > First Aid > Repair Disk Permissions. you
should do this at least monthly. This is sort of like the old
Rebuilding your Desktop. While you're at it, you might also check
Verify disk just to be sure all is right with the HD.
2. If you haven't shut down your computer recently, it sometimes helps
to do a restart.
3. Of course you should check to see how full is your HD.
4. Check your desktop. Is it cluttered with lots of icons? Each icon
takes up memory. If your have a lot, then just create a new folder and
call it something like John's Desktop and put all those icons (except
your HD folder), into the new folder.
5. Look at your Dock. Do you have a lot of Apps open? If you have some
open that you're not using regularly, go ahead and close them - they
take up processing memory.
6. Applications > Activity monitor is a good thing to look at
periodically because it shows you what is consuming memory and
processing power. But don't keep it open because it hogs memory too!
7. If all this fails, then go to this web site that has a bunch of
other things you can check: