How to Become a Great Rap Performer (from wikihow.com)

Steps
Practice facial expressions.
Don't let the crowd see that you're nervous even if you are.
Try to connect with the crowd, make them feel special.
Make eye contact.
Move around a little, don't be stiff.
make a distinctive style. all the greats have one.
never hold the mic down to your side. it gives feed back
give them an experience they will never forget. let the crowd in to you.
start off with a hype song. everyone would know you for.
Don't be predictable.
you have to be excited. the fans aren't going to be excited if your not.
wear a jacket or clothing that can be thrown in to a crowd.
good time to promote yourself if your not already known. pass out flyers or make t-shirts wit your song or album name on it
give a freestyle, or something they never heard.
swagger is a must.

Tips
Practice performing in front of friends,family, etc.
Practice performing songs of other artists.
Practice everyday

Warnings
Don't try to steal another rappers style.
Don't do anything stupid while on stage.