Buying drinks is definitely a bad idea as an opener. Subconsciously that is saying to the other person "you're so much better than me that I have to give you stuff so that you'll talk to/sleep with me"
Is that only if it is a woman asking to buy a man a drink? or is it the same in reverse because I honestly never thought that when a man offered to buy me a drink it was a self esteem issue.
Anyway I wasn't keen on asking to buy someone a drink anyway, I like my money and don't like spending it on others for no good reason.
Going up to someone, being a friendly person that people like being about, having a good time and seeing if they want to join in is basically what you need to do.
Stuff you could do to open conversations:
Comment on stuff:
"that's an interesting belt/tie/tatoo" (only if it is actually interesting)
Isn't this just general chit chat? because if not it must appear that I am hitting on men and women all the time.
In an ironic way, use cheesy guy lines:
"So do you come here often?"
"what's your star sign?"
"If I said you had a beautiful body, would you hold it against me?"
"How you doin'?"
*scribbles notes furiously*
As a start, I would suggest actually going up to a whole pile of guys and using cheesy guy lines just for your own amusement (i.e. not to hit on guys, just to get some conversations in. Note it has to be something you do for fun - if it is a chore, it will give you a bad vibe)
Like a whole pile of guys in a group? that would be far too intimidating for me lol.