I think it's different with every venue. Some venues are really strict, and they have shiploads of security everywhere.

But those are usually real big venues. And sometimes the way the venue is set up is not condusive for meeting anyone. Its all enclosed behind gates and what-nots.
The best venues i feel are the best to meet the band are small venues.

What one does is, after the show, go to where you see their tour buses, or find the back of the building. There will usually be other people waiting around to snag the band when they come out of the building, cause you know they gotta come out sooner or later!

i think people know that, so that in itself creates the meet and greet. Usually one has to wait for about an hour or so, so one needs patience and not care if you get to bed really late that night.
So once the band comes out, you should make sure you have something for them to sign. And a pen of some sort incase they dont have one. And hope and pray their managers dont scurry them onto the tour bus so nobody can even say boo.

...like they did with Jack White when i saw the Raconteurs last Friday night.
And also you could have unexpected surprises after a show. After the Raconteurs show, i hung around a little at the Denny's restaurant, and low and behold i spotted Meg White!

...and i got her autograph!

she's such a sweetheart.
So you see, after the show ya just gotta hang. Dont go home right away, you'll never meet anyone that way!
I have no idea how to meet the band before the show, nor do i know how to meet the band inside a venue.

Usually those people have VIP passes.