One of the worst ways a man can approach you in a nightlife setting is to come up behind you and tap you on the shoulder, or grab your arm through the crowd to get your attention when you aren't noticing him. It is abrupt, catches you off guard, puts you (and him) on the spot, and offers no easy exit.
The image below clearly illustrates the technique. In addition to the placement, notice the way the girl's bodies are angled in an effort to be more open to men who might approach. This "open" body language signals to men that you are open to meeting new people (usually they notice it subconsciously - but they notice).
As the night progresses and the bar gets more crowded, there will be more pressure pushing people into your "vacuum," and you will find yourself having to move around from time to time in order to always be posted up next to an empty spot. This can sometimes be a good thing since it gives you an excuse to spend time in different spots and be seen by different men. If the bar gets too full, you can apply the same principle as you stand overlooking a railing or balcony, though not having the drink transaction working for you makes this slightly less desirable. Keep in mind that staying in bars too late is asking for trouble, so I am assuming here that you will be gone before this becomes a major problem.
This isn't a magic trick for picking up guys. It isn't going to actively attract men. It will only facilitate the approach of the ones that are already interested. Rather than trying it out this weekend with grand expectations, I suggest making it a habit that you and your best girlfriends do by default, without expectation, every time you go out.
1. Other Nightlife Tips
2. Why You Don't Get Approached By Men
3. How To Make Yourself Approachable
4. Bars Are a Good Place to Meet Guys - Part 1