The Wall Street Journal took a look at tax loss harvesting to mitigate taxes and increase returns.

Our first interesting finding is that conservatively, investors can juice their returns 1.10 percentage points a year on average,assuming a 25% tax rate. If investors are pushing it in terms of taking advantage of every tax-loss harvesting opportunity, they can add as much as 1.42 percentage points a year to their portfolio’s return.

Not surprisingly, investors can reap the most from tax-loss harvesting when the stock market is having a down year. During years when the S&P 500 yielded a negative return, we estimate that the average benefits of harvesting losses amount to 3.21 percentage points of additional return to one’s equity portfolio (assuming a 25% tax rate). 

Just how valuable is tax loss harvesting? – WSJ