This is why all the retirement advice is to regularly contribute - Smaller amounts monthly (or even more frequently) works way better than only doing so when you're flush. The key to this is not spending all the dollars you make on your present lifestyle, but setting aside some percentage for investment from every paycheck. This means you'll be buying in the down times too, where everything is on sale.

Of course it also means accepting a lower spending rate in one's life.

You still end up ahead. [1] Even if you invest at the worst possible times, investing early and over a long period of time will still make you wealthy. This isn't necessarily the case in all situations; I think you would have lost money in the Japanese stock market over the past decade if you don't consider dividends/potential deflation. But it has held for the US market for a long time.

[1] http://awealthofcommonsense.com/2014/02/worlds-worst-market-...

You are wrong.

Start here - https://www.bogleheads.org/

You're wrong. If you only invested lump sums in the S&P 500 in 1998, 1999, 2005, and 2006, you'd have made money by today. Not as much as you would have historically, but still a fair chunk - and while yes, inflation would have eaten a large, large portion of it, it's way better than leaving your cash under your mattress.

There's no way you lost money if you were in the S&P 500, unless you were pulling it out of investments after crashes - in which case, duh, you're buying high and selling low, what did you expect?

And that's if you were only in the S&P 500... a more reasonable retirement portfolio is much more diversified, which would have increased your returns and lowered your volatility.

http://stockcharts.com/freecharts/perf.php?SPY

Asset prices are depressed during bear markets. Those make incredibly good times to buy, because when the market jumps back up your dollar during bad times becomes so much more valuable when those assets appreciate during good times.

If anything, during good times it may make the most sense to store value in liquid accounts and wait for fire sale prices when the economy craps out. But some consistent growth is better than none, due to dollar cost averaging (i.e. you don't know whether asset prices are going to go up or to go down) [1].

[1] https://en.wikipedia.org/wiki/Dollar_cost_averaging

Real Estate.