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.