Just
Plain Good Sportmanship/Sportswomanship
Don't drop or rattle your darts.
Don't
talk to him/her, tell him/her their score or the number he/she
needs.
If he/she wants help, they can step off the line and ask
Limit
encouragement and gamesmanship to before he/she steps to the
line
and after his/her turn is over.
In
short, do nothing to break a player's concentration while he is on the
line.
This
applies to opponents and teammates alike.
Have
fun and a good time, but remember your manners too!