Play primarily chords while you accompany any singer. The singer will provide the melody. When you accompany on the piano, your piano should function in a supporting role.
Don't overplay. Pianos are strong instruments that can easily overpower a singer if the pianist's playing technique is loud. There will be times when playing loud or forcefully is part of the song. Limit your aggressive playing to those times.
Lead the singer without making it obvious. Playing the starting note of each vocal line can enhance the melody, and help cue the vocalist without being too obvious.
Double the root note of the chords you place to give weight to the chord progressions. This creates a more effective tonal foundation for the soloist.
Create musical fills based on the melody and chord figures when the soloist isn't singing. This is the time for the piano accompanist to shine.
Maintain a study of chords. Chords are the main tools in your repertoire. The more chords you know, the better equipped you will be to sit down and accompany on a moment's notice. It's a good idea to keep a book of piano chords or an online chord dictionary for easy reference and study.