This is the ultimate question, please let me know when you have it nailed down :) Volatility is associated with risk, so in order to reduce volatility, you have to focus on eliminating any risk exposures that you are not intentionally betting on (your source of alpha).
There is no cookbook, but a common method is to take long/short offsetting positions in 'correlated' assets. Pairs trading is the classic example of this. In this algo I paired AutoZone (AZO) with O'Reilly Automotive (ORLY), both companies have similar business models, and are exposed to the similar market risks, so they should share a similar trend.
In this case the only risk I have any view on is within the automotive retail business, but the auto biz is still exposed to broader market risks like anything else. In order to isolate the risk that I feel I have insight into I buy one and sell the other in equal $ amounts. That way I don't care what the market does, just how ORLY is performing relative to AZO.
Keep in mind that this kind of investing methodology is not meant to 'beat the market,' it's a slow and steady wins the race thing, the goal is to not go down with the ship. This pair underperforms this backtest in practice (maybe worse now if readers trade it) because the bid/ask spread can get pretty wide on these stocks, but the principle remains.
Long story still long, look to eliminate/isolate risks you have no knowledge about. A portfolio of pairs can work well if you are creative with your selection and put your own spin on the idea. Beware of low returns and abrupt losses due to earnings and other unforeseeable events though.
David