I believe Daftek is correct. Many people fail to meet historical returns because they try to pull out before a down and then put in before a rise. You would have to sell tomorrow if you think it will fall, and then you would have to know when to buy back in. You will most likely take some loss during the sale, and you will most likely lose some gains trying to get back in at the right time.
If you continue the behavior of selling on a down and buying on an up, it is more likely that you will shave lots of points off the historical returns rather than boost them.
For most humans it is much easier to leave in for the long haul to capture the historical returns. If the capital losses cause you to freak out, it may be safer on your sanity to stay out of stocks.
Personally, I would freak out if I had to try to time the market without having some secret information that I knew was going to come out. Everyone already knows the market is volatile right now. It is no big secret.