There are different approaches to protecting profits and reducing your losses in a bear market and they offer varying degrees of protection. Some will even make you money.
First, you have to differentiate between a crash and a bear market. 1929 and 1987 were crashes. Unless you had some form of negative correlation protection in place prior to them, you paid the piper. Bear markets like 2000 and 2008 took 18 months to unfold so they offered lots of time to react and adjust.
The buy & hold types recommend diversification and reallocation. AFAIC, that's just a way to spread the losses across various sectors, hoping that some don't lose as much as others. But you will lose. Would you consider losing 30% in 2000 or 2008 being protected when the market lost 50+ pct?
Some will recommend gold. Sometimes gold correlates, sometimes it does not. It's an iffy proposition for protection.
Some will recommend bonds. Like gold, they don't always move inversely to the market and given our historically recent years of low rates, they may again correlate with the market (both go down).
Some will recommend stop loss orders. That's fine in a correction but in a crash, you have no clue where the fill will be if price gaps through your stop loss price. Again, iffy.
You could write covered calls but that provides only limited downside protection and if your stocks crater, you won't be able to continue doing so without locking in a loss.
Others will recommend buying puts. That offers several problems. Even at the current level of implied volatility, portfolio hedging might cost 5–6 pct a year, depending on the index. That's an awful lot of portfolio drag to overcome if the market stagnates or moves up modestly. You can reduce put protection cost if you use OTM puts but that adds a 'deductible' to the put cost which is the loss from current price down to the put's strike price. If you buy puts and the market moves up, the additional gain is not protected.
If you are comfortable with giving away much of the upside potential gain, you can reduce/eliminate the put cost by collaring long stock. Sell OTM covered calls to fund the cost of the protective puts. Note that collared long stock is synthetically equivalent to a bullish vertical spread. Verticals have a limited a R/R profile.
There are other sophisticated approaches:
1) Buy inverse ETFs
2) Run a long/short portfolio
3) Short stocks, ETFs and futures
AFAIK, no one can predict when a market will crash or correct significantly. Many will guess and a few will be correct. The wiser approach is to recognize that severe market downturns such as the 2000 Dotcom Bust and the 2008 subprime melt down did not happen overnight. For those who aren't oblivious, along the way it becomes obvious that market metric are deteriorating - economic indicators turn down, earnings announcement disappointments increase, analyst downgrades and earnings revisions increase, the VIX increases, all beginning long before the crisis becomes acute. React, don’t predict.
Transitioning to cash or quality debt is something that any investor can achieve. If god forbid one takes off the 'long only' tunnel vision blinders, one can even go short a small position and transition to more short as the market drops further. Let your portfolio’s declining value dictate the transition from long to short (and vice versa).
My vote for the average Joe investor is to collar your overvalued positions. You'll still have some upside potential but you won't get hammered if there's a large correction. If your collars are 2–3 months out, if the underlying cooperates and appreciates toward the short call strike, you’ll be able to roll your collar up, locking in the appreciation and raising the level of protection.