'Hi there, long time no see' is very casual. If it was your boss, you might say 'Good morning Jim [ or Mr Jones if you are formal in your company]

saying hello to others is a very casual greetings even though you dont know the person your saying hello its a sign of respect and courtesy to the person...but how you will gonna say it? thats the question, well for me it depends on the circumstances, the mood and ofcourse depending on the customs and traditions of the person you will say hello for here in italki we are worldwide so we have differences..so probably the proper way to say hello is..just say it within your heart and express it as if your already known the person for awhile..meaning say it in friendly like manner..

It depends on whom you are communicating with, if its a casual friend, you can simply say, "What's up" or "How are you doing." If you are meeting a friend after long time, you can express your delight by saying something like "Is that you?" or "what a surprise, it's nice to see you after such a long time." You have to reinvent yourself every time to come up with the perfect phrase and that is the fun of it.

hi,what've you been upto?...i prefer "whatsup?"....wazaaa:))

When I'm speaking formally, I use: -Hey/hello/hi, how are you? -hi, how have you been? To be casual I'll use: -how are you? -what's up? -how's life? -how's life been going? -watcha(slang, which is incorrect grammar by the way. short for "what are you") up to? Other greetings you can use (informal or casual): -wazzup? -dude, what's up -hey man, what's been kicking? -what's cracking? -what's happenin'? -yo -how you doin'? .....Those are just some examples. I'm sure there are more greetings.