I wouldn't really consider those crashes! They are Bear Markets. Look at the time periods, most are between one and three years. If you invest regularly, you will buy on the way down and on the way up, and when it is over you will still make out in the end. Don't think of them as doom and gloom, think of them as discounted.
I am willing to bet if you look at then next 2 or 3 years following any of those Bear Markets the charts would tell you a great story of gains!
It is a cycle, a good cycle if you are long term. I only lost about 20% or so durring the 2000-2002 period but since late 2003 to now my total investments have more than doubled.... they are getting close to trippling. Why? Because I didn't panic, I played it cool, and I was constantly buying durring the last Bear Market.
Rember to look at the BIG picture!
Here is a chart from 1970!
Here is one from 1930... notice that the farther back you go the less signifigant the bear markets are.
Here is one from 1920