Automated Stock Trading Methods

Single stock based:

Sell when the stock is up N percent and buy it back when it is down N percent?

Sell when a short term moving average crosses down a long term moving average and buy it back when the short term moving average crosses up the long term moving average?

Sell when the price-earnings ratio is up N percent and buy it back when it is down N percent?
(Multiple indicators similar to PE ratio can be multiplied together and can be used in similar way too.)

Multi stock based: (More suitable for index funds?)

Multiple indicators similar to PE ratio can be multiplied together and can be used to calculate a score for each different stock.
(If an (normalized) indicator is higher the better kind then multiply, lower the better kind then divide.)
Then sell N worst ones and buy N best ones every M days?