A few weeks ago I bought Home Depot (HD) for $29.02 and I bought Coke (KO) for 52.91.  They both went way up.

we'll say I bought 100 shares of each, just to make the math easier.

So, let's start with Home Depot.  

$29.02 * 100 shares = $2902 invested

Today I sold some shares at $34.46, but lets say you sold all 100 shares:

$3446 (Todays stock value) - $2902 (the amount you invested) = $544 profit.  * 30% short-term tax = $164 in taxes

Lets say instead of selling all of the shares, you just take out the profit.

$544 (profit) / $34.36 (current stock price) = 16 shares (rounded up)

Sell 16 shares at $34.36 = $549.76 in cash and I still have 84 shares left.  Here's the best part though, I only pay taxes on the amount those 16 shares gained.  So:

$34.46 (todays price) - $29.02 (purchase price) = $5.44 a share gain.  Multiply by the 16 shares we sold = $87.04 taxable * 30% (tax rate) = $26.11 in taxes.

So, both ways you took the profit, however the first way is going to kill you in long-term investing.  The first trader has $3282 cash and no stock after taxes.  The second trader has 84 shares left at $34.36, plus the roughly $524 cash they cashed out after taxes, for a total of $3410.24 roughly.  Quite a big difference, and this is with only 100 shares.  You could see how it would become a huge difference if you had a lot of shares.

With Coke I did even better.  

Selling all shares $6460 (todays value) - $5291 (amount invested) = $1169 taxable * 30% = $350.70 in taxes.

Selling partial shares $6460 (today's value) - $5291 (original value) = $1169 (profit) / $64.60 (todays share price) = 18 shares to sell.

$64.50 (today's price) - $52.91 (original price) = $11.59 profit a share taxable

18 shares * $11.59 (gain) = $208.62 taxable gains * 30% = $62.59 in taxes, not to mention I still have 82 shares of Coke remaining invested.

The best strategy from here is keep those gains and buy more shares on dips of the same company.  You can amass huge numbers of shares this way, without using your own cash.  Remember, the key to making money in the stock market, IMO, is the NUMBER OF SHARES you have, not the amount a stock moves!  Amass shares, sit on good companies with name monopolies and you'll automatically be in the money.

Invest in peace...