Practice should help; there are debate clubs for that. Or try debating politics - it's mostly useless, but you'll never lack willing opponents. Knowing your sub

Practice should help; there are debate clubs for that. Or try debating politics - it's mostly useless, but you'll never lack willing opponents. Knowing your subject very well helps, too. Staying calm is always useful; I have no recommendation on how to achieve that, though (I never really had to work for that.)