There are so many variables in this. Some bands show up really early and hang at the club all day, some don't show until they are on. After the show, some rush out, others linger. You can look for the thread on Massive Attack and someone tells how they got to meet the band, some good pointers there but it is usually luck.
Also sometimes they will be at the T-shirt counter after the show, or just come out to pack up their gear and are often approachable.