why you'll never be rich (a slide show from Kiplingers)
Or, more positively, 10 things to avoid if you want to get rich.
and buried in there, you might want to check out the 7 Deadly Sins of Investing.
and, on a related note, 28 ways to waste your money
If I had to choose just one rule, it would be from Andrew Tobias' The Only Investment Guide You'll Ever Need: spend less than you make.
Consider the words offered to Charles Dickens by his father .. "Annual income, twenty pounds; annual expenditure, nineteen pounds; result, happiness. Annual income, twenty pounds; annual expenditure, twenty-one pounds; result, misery." That's pretty much it. Spend less than you earn. Live a little beneath your means.
[6/18/14 - the above is in the revised 2005 edition of the book, I don't see it in my second edition printed in 1986. Maybe it's in the copy I gave to Timmy.]
Googling, there are others saying the same thing.
Rule #1 (by Trent) [not to be confused with Rule #1]
Get Rich Slowly