The holy grail of diversification is to find several uncorrelated asset classes all with positive returns. One problem, though, is that diversified passive investing has caused all asset classes to become more and more correlated over time. Increasingly, you see stocks, bonds, commodities, and cryptocurrencies all move together.
One approach to diversification that's increasingly popular with quants is to diversify your strategies rather than your asset classes. Long-short strategies are a popular example. Almost by definition, your short strategies will make money when your long strategies lose money, and vice versa. The challenge of making this work is that it's really hard to design short strategies with positive expected return. Since the market tends to go up over time, playing the market short is a bit like betting against the house at a casino. If you find a short strategy that actually works, that's gold right there.
Fortunately, there are some relatively uncorrelated strategies that work for long-only traders. This chart shows the Invesco "Momentum" and "Pure Value" ETFs. As you can see from the red and green arrows, the two ETFs often move in opposite directions. When one is producing positive returns, the other often isn't. Owning both can help smooth out your drawdowns and returns.
The same can be said for "mean-reversion" and "trend-following" strategies. Mean-reversion strategies involve buying assets that have made a big move downward. If you bought China stocks after their recent huge-selloff, that was a mean-reversion trade. Trend-following strategies, by contrast, involve buying assets that have made a big move upward. If you've bought oil and gas stocks in recent weeks, that was a trend-following trade. Both strategies tend to "work," but again, they're somewhat uncorrelated.
These strategies can further be broken down into short-term and long-term versions. Oil and gas is in a short-term uptrend, while the Nasdaq index is in a long-term uptrend. Facebook and Bristol-Myers Squibb are a short-term mean-reversion candidates after their recent sell-offs, while Calavo Growers and Regis Corporation are long-term mean-reversion candidates. The nice thing about using a mixture of short-term and long-term signals is that they allow you both to profit from stable market conditions and to quickly pivot at least some of your capital when market conditions change.