All you need is an IDEA.
I have been operating largely according to principles of value investing, while occasionally I deviated and tried to catch short-term momentums (which to me are more like anomalies). However, this is also where I learned lessons from my deviations: once you formulated your own principles, never deviate from them.
I have found many occasions that when I deviate, I would generally be wrong and suffer undesired losses.
Reality has taught me that you cannot outperform any computer algorithm in trading in short term, since machines are so much quicker and more precise in computing and capturing moments than human brain, not to comment on human emotions and subconscious decision-making mechanisms. However, if you stick to your principles in the long term, it is absolutely possible to outperform those machines, since they are simply digesting data that are generated by human actions and spitting out whatever optimal solutions they deem fit, again based on these human trading data.
In a word, garbage in, garbage out. After all, it is those humans that formulate those algorithms for the super computers, and indeed, since they are man made, they are still prone to errors.
Conclusion: don't try to play short-term tricks with computers, outsmart them in long term with strict and well-formulated principles.
I know you have the same feeling as I have: whenever I sell a stock because it does not perform, it often turns out that It begins to take off after you sell it. I've seen it many times and heard many times from my friends, as if the market is a calculating machine that works against you. In a way, it is. But mostly, it's because we are human beings, and we want quick profit, and we lose patience very easily. Many of my holdings turn out pretty well because I hold them long enough. Just like you find a gem in a debris of garbage: at the beginning it looks ugly and stinky as the other garbage in the debris, but with time, when other people start to realize it is a gem, they will do the polishing for you. And with the polishing, still more and more people realize it's gem, and there comes your payoff. But of course, that takes time; and, more importantly, you have to convince yourself that the garbage you found is indeed a gem. This requires a discerning and well-trained pair of eyes and a lot of patience. Do both well, and you will get rewarded.