Always the bar, never the pin. Unless you want a perforated tie collection.
It should not only clip together the two ends of the tie, but also secure them to the shirt's placket.
Position the tie bar between the third and fourth buttons of the shirt. Any lower will obscure it behind your jacket; any higher looks peculiar.
The bar should never extend across more than three quarters of the tie's breadth. If you've acquired a super-skinny tie you need to downsize your clip. No excuses.
Avoid bling. The bar should enhance not distract (and if it's vintage, even better).
Originally published in the August 2012 issue of British GQ.