When is it proper to press in a round of golf?
A collared shirt and either shorts which for men are not too short or slacks. Ladies sometimes wear skirts. Jeans are not preferred..
If you're going to press, you need to announce it after the completion of the hole (so everyone knows you've called a press before they tee off). Typically, you'll sort out your ground rules (I have an occasional game with some sticks who have automatic 2-down presses along with an automatic double-or-nothing on the 18th hole) so you know what's at stake BEFORE you tee off (and what you're on the hook for).
