Make sure you are prepared. Ensure your batteries are fully charged and bring extra blank media and batteries. The concert could go much longer than you expect and it's not much of an inconvenience to be prepared. Check with the venue and performing artist to verify that they allow recording of their shows. If they don't, your equipment could be confiscated and you could be thrown out of the venue.
Try to get to the concert early so you can pick the best spot to position your recording equipment. As soon as you arrive, try to locate the speakers that will be amplifying the music. Position yourself about 20 or 30 feet from the speakers, but try to maintain a direct line-of-sight connection with the speaker. Anything between the microphone and the sound source will muffle the sound.
If you are using a directional microphone such as a cardioid or hypercardioid, it's best to try to keep the audience behind the microphone. This is because directional microphones pick up less sound from behind them, which is a useful feature when you don't want to capture someone's loud conversation on your recording.
For best sound quality, keep the microphone above the audience as much as possible. Again, a person's body between the microphone and the speaker will dull the sound and can really ruin a recording. If you are using a directional microphone, try to move it around as little as possible. The sound can get slightly warped if you move the microphone too much, resulting in a very odd listening experience.