I would usually call the waiter by trying to catch his eye or raising my hand eventually. But I suppose there are other customs depending on the region of the world. What are those? Is snapping ones fingers always considered impolite?
In Western countries, it's usually best to be discreet about it. When in doubt, always look around and see what the others are doing, in a normal-sized restaurant there would be always people around ordering and clamoring for attention, so get some clues from there as well.
Generally, I'd usually try to make eye contact the waiter and communicate my desires without them having to come around, i.e. if I'd like to ask for the check I'd make a discrete gesture as if signing a paper, most waiters (especially in restaurants) would understand what I mean, even if we don't quite speak the same language. If I just want to order more of the same (most likely, beer or wine) I'd point to it and raise a finger or two. This applies even more for busy places where waiters are always running around, and their efficiency is key.
To draw their attention for other purpose, I'd slightly nod upwards and maybe raise two fingers slightly when I'm sure they see me, that usually does the trick.
I wouldn't raise my hand unless waiters continuously ignore me, and I wouldn't raise my voice to be heard -- I'd speak to them when they pass around me. However, inordinate waiting times would reflect on the tip I leave in the end, if any.
I also make it a point to always be polite and considerate with service personnel, even in countries where some degree of servitude is common.