Here are some golden rules to follow
1. In 3NT, always try to put down first the suit they have led. Gives partner those extra couple of seconds to work out his play in this suit.
2. In doubled contracts and slams, show partner the good news first.
3. Stay at the table. Avoid the Versace walk.
I am also told that when some good bridge professionals are forced to be dummy, they place either shortage next to the trump suit (tip: ruffs) or their side-suit (tip: set it up).
The contract is 4H on the lead of spade Jack to Ace and a low spade is returned.
Which suit would you place next to those trumps ?