I love it when women buy me a drink. It's a great icebreaker, and a step outside of the norm. It's also a clear sign from the woman about her intentions, which is a rare and beautiful thing. It'll likely catch him off guard, but I don't know anyone who would refuse a drink from a girl they were talking to for hours at the bar previously.
On another note, If you want to be more successful at flirting it's really not that hard. Make periods of consistent eye contact (not extended, that's domineering/creepy). Initiate physical interaction by touching him (usually on the tertiary parts of the upper body [hand, forearm, elbow], to avoid scaring him off). Compliment him on parts of his appearance or personality. ("have you been working out lately?", "What are you all dressed up for? You look good.", "You are so funny!" etc. [Mind you, you should be more creative than me]) Ask him about his personal life ("do you have a girlfriend?", "So, do you live alone?" , etc.). If you've done this, and still feel like it's not working, he might not be into you, he might not be available, or he might be shy. Any which way, it's probably better to take the slow and cautious route.
Also, don't show up with a friend who is a guy. If you are hanging with a dude all night, it makes you off limits immediately, and most men worth their weight in lead won't go after some other dude's chick. Anyone that would, is an a**hole.