It helps when the girl is so drunk she throws up very soon after making out with you. Not that I was sober either.
No way. This is the best way to get yourself quickly cock-blocked by her friends.
It helps when her friends are drunk too?
I'm sure it's a little different if you're really, really, extremely good-looking, but in my experience, there's always one or two girls in the group who are either fat or PMSing who really don't want any of their friends to get laid that night. If one of the girls gets really drunk, you can bet that the fat/PMSing friends are going to be keeping a close eye on her to prevent her from making any "bad decisions."
If the entire group gets wasted, then you're in, but it can be a challenge getting to that point, especially if you find yourself having to buy drinks for the fat friends. Having a good wingman who will take one for the team is crucial.