While I was traveling last week, I received an email that made me think long and hard about one of the more difficult tasks for an investor: selling. Because this reader's email asked about a couple of Bottom of the Barrel stocks, I figured it was time to think about the process of deciding when to sell your small-caps.Compared with deciding when to buy, knowing when to sell is a chore. Hundreds of theories exist about when to sell, and all of them are right and wrong, depending on your investing philosophy and risk tolerance. Ironically, selling discipline is always more difficult than buying discipline, largely because selling is always remembered. Just think about the sales you've second-guessed and compare them with the bad buys you remember. The sales stick out more vividly, at least more often than the reasons why you bought the stocks in the first place.