On when to sell ... Irish makes a good point (the best point actually), that is to take profits no matter what. I experiment with different methods depending on the volatility of the stock. Use GTC's. Immediately after buying a stock I might place a GTC to sell 80% at 20% gain, or 70% at 40% gain, or 50% at 100% gain, etc. Stay diversified (buy good stocks and ride the dips), preserve your capital, and collect free positions.
I've also been known to go ALL IN into one momo stock and sell for a quick 10 or 20% ... not reccommended for most, but is quite beautiful when it works. Most people struggle all year to show those returns. Given the right opportunity it can be done in about an hour, or less. lol