Don’t set a deadline unless its real.
Don’t make someone kowtow just because you know he needs your account.
Don’t be nasty to suppliers.
Maybe you’ll get away with it for awhile, but eventually your best suppliers will find other customers, and you’ll have a reputation for being a terrible client.
The world goes round in circles.
Be considerate and polite to your suppliers, who knows, maybe one day they’ll be your clients.