I've started reading Benjamin Graham's book The Intelligent Investor and have been amazed by some of the insights that he had.
On page 203 Graham writes "The true investor scarcely ever is forced to sell his shares, and at all other times he is free to disregard the current price quotation. ... Thus the investor who permits himself to be stampeded or unduly worried by unjustified market declines in his holdings is perversely transforming his basic advantage into a basic disadvantage."
What Graham is saying here is that you are rarely forced by means outside of your control to sell your shares and that you shouldn't get caught up in the crazy swings of the market. Don't see a massive bleeding on the stock market and take that as a sign that you should sell out your positions. As long as there have been no fundamental changes with the underlying companies then sit back knowing that you are getting more shares from your dividend reinvestment and you should consider adding to your position. The market is one crazy s.o.b. and you should ignore the short term price fluctuations.
I highly recommend reading through The Intelligent Investor if you haven't done so already.