When the band is done performing, they will have two things on their mind. Getting laid and getting intoxicated. If you are an attractive and easy going person then offering sex may get you invited. If you're a slob like me, you might want to try booze. Offer to buy a round of beers. Be prepared and say "Hey guys, I got a hooker boat (18 pack) in my car. Where's the party?" Some musicians may be looking for drugs. I speak from experience when I say drugs are very dangerous and you don't want to get involved with them at all. Don't bust out an eight ball or someone could end up dead or institutionalized.
Always be friendly after a show and thank the band personally. A quick way to find the after party is to just ask. It may be at a bar down the street that serves stiffer and cheaper drinks. If this is the case just follow the crowd.
Not every band has the ability to have a house party after a show. If you have a well stocked and empty house you could always offer to host the after party at your pad. Just make sure the neighbors are cool because it could get loud and rowdy. Also put the fragile stuff away because your place might get trashed! If your house is big enough you might even be able to talk your new musician friends into playing a house show.
This last tip is what got me invited to all the after parties. If you got the equipment, videotape the whole show. Musicians are self conscience and they are gonna want to know if they killed it onstage or totally sucked. Bring your cords and let them know you can plug into their TV and let them watch the whole show. You will be a hero and instant friend.
Use these tips and you will be partying with rock stars in no time. Some of the friendships you make could even turn into business partnerships if you have any interest in show biz. I had the time of my life and I bet you will too.