The key word to Yemen custom could be modesty. Dress modestly. For men this means: do not wear shorts in public, wear a shirt with long sleeves - maybe rolled to the elbows. For women this means cover your legs and you arms along with the rest of your body. Not only is this the best way to get yourself accepted in a country were women are covered entirely, and men wear either long pants or a fouteh under a long sleeved shirt and a jacket. It also is the most certain way not to die of sunburn.
And act modestly. Show that you are aware of being a visitor in one of the oldest civilisations of this earth, and most people will give you a warm "WellCoome", or at least a sincere Salaam Alaykum.
Special tip for non-islamic visitors: Don't get drunk - especially not in public. There is no way to compensate the loss of face. Keep your duty free liquor for late at night - in your hotel room. Or even better: keep it for home.
And don't even think of taking a picture of a woman. Unless she asks you.
One local custom that can not remain unmentioned is chewing qat.
After lunch a large portion of the male population goes to the local qat market to purchase a bagfull of leaves, which are chewed for the rest of the afternoon and the evening.
The effect is reputed to be mildly intoxicating, and any form of addiction is denied.
The visible effect is that qat chewers are speaking at the same time more loudly and less clearly, get a mildly unfocused look, and become tremendously relaxed and easy going.
Once you get to used to it, you hardly notice that shopkeepers, taxi drivers and passers by are all mildly stoned