"Investors should remember that excitement and expenses are their enemies. And if they insist on trying to time their participation in equities, they should try to be fearful when others are greedy and greedy only when others are fearful."
Source: Letter to shareholders, 2004
Read more: http://www.businessinsider.com/warren-buffett-quotes-on-investing-2010-8?op=1#ixzz1oXFcQIec