In a nut shell you can't. The only thing you could do is replace the hard drive. It will be getting old now, say at least 7 years! Its bound to have a few issue.
If you do decide to get the drive relaced do not attemt this yourself. It is one of the hardest hard drives to replace on any Mac. Theres 4 different layers to remove and about 30 screws. I repair Mac's for a living and I used to hate this repair job. Used to take a good hour and Im an experianced engineer.
If you do want it doing try to find someone with good experiance of these and will not charge you through the roof. We are based in London but Im not sure iwhere in the world you are! Might be able to recommend somebody.