Casual greeting in Japan

A Lang-8 user asked me how to greet on casual ocassions in Japanese.

It's a bit difficult question.
I didn't think of any specific casual greeting like "hi" or "salut".
In case of work place or stiff occasions it's different, but with friends... I can't think of any proper greeting.

In the morning, you can say "Ohayou = good morning" to your friends.
Or if you meet a friend after certain separation, you can say "Hisashiburi = Long time no see".
But except those occasitons, I think that many people (especially youg people) start on conversation without any certain greeting.

Maybe it sounds strange for you that Japanese don't have casual greeting.
But I think that the idea about occasion in which you have to greet is different between Japanese and others. (Maybe it's different in each of the countries.)

For example, for me, it's a bit uneasy that there are no certain greeting for beginning of meal or at coming home.
Before beginning of meal, Japanese say "Itadakimasu = Thanks for having meal.".
At comeing at home, Japanese say "Tadaima = just now = Now, I'm coming home.".
I've already asked my friends and my language teachers about that matter, but they said that there is no specific greeting for these occasions in their languages.

For another exemple, in Japan when someone enters into a shop, shop staff greet customer but usually customer says nothng.
I think that in Europe when someone enters into a shop, customer has to greet first then shop staff greet back.
I'm afraid but sometimes Japanese tourists feel European shop staffs to be impolite. I assume that European should feel Japanese to be impolite... ToT