1. The most important thing to remember about accompanying a choir, singer, or instrumentalist is to LISTEN! Since you're not the main role in the performance, you must follow the other musician's interpretation. Watch the conductor very carefully when accompanying a choir.
2. Don't forger to play with feeling and sensitivity. That means phrasing, dynamics, balance, etc. Sometimes you might feel that you're just an accompanist, but in fact you have a very important role to play.
3. Listen carefully (again) to make sure that you're not (usually) playing louder than the person you're accompanying.
4. Being able to sight-read well is a very valuable skill for an accompanist. You don't always get a chance to practice well ahead of time, so sight-reading is very important. Also, being able to read chord symbols or read open score is an asset.
