The only really sensible investment advice I know of came from Will Rogers, who is alleged to have said:

"You buy your stock.
When it goes up, sell it.
If it don't go up, don't buy it."

Advice I just don't get includes the following:

People are very optimistic; so stocks will decline.

People are pessimistic; it's a good buying opportunity.

Stocks go down in September and October.

You can't beat the market; it's all priced in.

You've got to do your homework.

Go with the momentum.

The Dow is 5½ points below Fair Value.

Sell in May and go away.

Cash is king; stay liquid.

If you're out of the market a few days, you can miss the run-up.

The trend is your friend.

It's a random walk.