A momentum strategy for the S&P 500 works. We use monthly bars and a simple moving average strategy crossover system.
We backtest the following trading rules:
When the close of the current month crosses above the 12-month simple moving average, we go long.
When the close of the current month crosses below the 12-month simple moving average, we sell (and stay in cash until we get a buy signal).
This is it — two simple rules. You can also enter at the open of the following month after a signal.
You can find more info about this trading strategy here:
https://www.quantifiedstrategies.com/sp-500-momentum-strategy/