Unfortunately most people, and most of the investment literature, only deal with what stocks to buy and when to buy them. They almost never mention position sizing and when to sell. It's a shame and we should really focus more on those 2 topics. IMO this is the importance of a trading system, in order:
1) The markets/stocks you are going to trade
2) Position Sizing
3) Exit strategy
4) Entry strategy
You'll notice that entries are the least important to me. You can have a completely random entry system that makes a decent amount of money, as long as you get the first 3 nailed down. This blows people's minds but it's true. Grab a quarter and do a coin flip, if it's heads you'll go long and if it's tails you'll go short.
Completely random. Use SPY as your trading vehicle (tight spreads, superb liquidity and easy to short). Enter and put your stop loss $1 away from your entry, and make your profit target $3 away from your entry. Although I would never trade this system (it will have horrible drawdowns), you'll be amazed that it actually makes money over enough trades. If a random entry system can make money, then just how important are entries in the first place? You can make it even more intelligent by using a trailing stop (like a trailing ATR stop) and let your profitable trades run as long as they continue to go in your favor, boosting your profitable trades considerably.
Van Tharp has some good info on position sizing in his books, Ralph Vince has some as well but I haven't read any of this books, just Tharp's. Most of the investment literature simply mentions diversification as the position sizing strategy, hold 10-100 stocks (at equal weight???) and don't worry about it further. But most of the really successful investors didn't hold their positions at equal weight. For their high conviction investments they may have put 25-50% of their portfolio into that one trade. And it made them a ton of money. I don't know how you could quantify that style into a repeatable system however.
Some things that come to mind that would be objective and take the emotions out:
1) A trailing stop like a moving average, trailing ATR stop, or some other volatility-based trailing stop
2) A system that automatically sells and buys cheaper/better stocks like the Magic Formula
3) A system that automatically sells and buys stocks with better momentum
4) For dividend investors, sell a stock if they ever cut the dividend
5) For growth investors, sell a stock if the story changes, it stops growing earnings, management begins to reinvest profits poorly (deworsification), is the management executing on their plan to grow? They usually say what they plan to do, if they aren't doing what they said they would do, that could be the signal.
6) Don't make it an 'all or nothing' deal. If the price doubles, take off 20% for each time it doubles. This allows you to keep a good chunk of your investment while taking some money off the table if it reverses back down. I believe this is what David Merkel at alephblog.com does. You can search for his posts where he gets into more detail, but the basics are here
http://alephblog.com/the-eight-rules-of-my-investing/