I know it breaks all the rules about diversification but rules are made to be broken. I suspect there are many "millionaires" here on HC who have done so.
I have no current tips but I am, like AP below, grossly overweight one stock and it has kept me alive during the GFC. I wasn't overweight in it in '07 but it was the one I had confidence in and never sold. Margin accounts are unforgiving and little else survived.
