how do u call over a waiter / waitress ?

Yeah you're right. If they are directly looking at you, a nod is okay already. I guess if the restaurant is really full packed and all are busy then maybe you can raise your hand just to attract their attention. No need to shout or anything like that. It's never nice to sound bossy. A friend told me that we should be nice to these people because we never know what they'll put in our meals :p... Is that true btw? just curious :)

Generally, I wait for my server to pass by the table and try to catch them by saying "excuse me" or I wait for them to return to ask if I need anything. Barring that, if the server is a distance away, I will try to make eye contact and, if I'm successful, I will raise a hand slightly to indicate that I'd like to see them. I really don't like when people snap their fingers at servers either. I think it's incredibly boorish behavior.