Make sure to determine what your cost basis is, especially if you've sold shares you bought over time. And if you've bought mutual funds and reinvested dividends, that affects the cost basis of your mutual fund shares. Ideally, if you sold shares during the year, you told your broker in writing which ones you wanted to sell and got written confirmation back. This is called specific identification. Let's say I have 500 shares of IBM that I bought at five different times.
Some shares have a much higher basis than others.
You can't pick and choose after the fact. It's too late for last year's investments, but this year people should think about using their losses to offset their gains. Normally I focus on this later in the year, but it's probably something to think about all year long. If you change your mind, you can buy it back in 31 days and still get credit for the loss. And don't forget your mutual funds.