New York When the market is plunging like it did last Friday and on Monday, it's tempting to throw in the towel and sell. Big drops can be scary.But dumping stocks on days when the Dow is getting whacked is usually the wrong thing to do. Stocks roared back Tuesday and were up again Wednesday.If you're investing for the long haul, the best thing you can do is ride out this wave of volatility."Stay invested," said Seema Shah, chief strategist at Principal Global Investors. Shah told CNN Business t