If you need to be formal or are talking to someone who is not a friend, stick with "How are you?" You can emphasize any one of the three words. I emphasize "are". It doesn't matter which one you emphasize, but you must emphasize one of them. Otherwise, it will sound like you don't really care what the answer is. Other expressions I use: "How's it going?" "How have you been (lately)?" "How have you been feeling (or 'doing') (lately)?" "How 'ya doing?" "What gives?" "What's (or 'what's been') up (with you)?"

How is life? How are things? How are you doing?

I agree with the other teachers and here's a few more: How are you doing? (sounds like "how ya doin'?" - rather informal) What's up? (informal)

‘How are you ?’ is fine. The first word ‘how’ is emphasised. You generally only use this with people you know well or see regularly.

How are you? is perfect. How's it going? Informal How have you been? (If you know the speaker, invites them to use the present perfect to summarise their life since you last saw them: 'OK, not bad, work's been mad!'