Taking capital losses is timing the market, but can be better than inaction. For example, Vanguard Total Stock Market ETF ("VTI") tracks a different index and has twice as many stocks as Schwab US Broad Market ETF ("SCHB"). If you paid more for VTI than today's price, after 31 days you can recognize a tax loss. You sell VTI for a loss, and your taxes will reflect a deduction for the loss. Then to avoid being out of the market, you immediately buy the same amount of SCHB.
I think the biggest thing is finding similar pairs of ETFs that roughly match, but avoid the IRS wash sale rule for "substantially identical" assets. Here's my short list:
VTI ~ SCHB (Vanguard vs Schwab Total Market)
VEA ~ IEFA (Vanguard vs iShares international)
VWO ~ IEMG (Vanguard vs iShares emerging markets)