Regular tunings are the main additional expense. Twice a year should be sufficient for most environments. It is important not to neglect this as pianos will inevitably loose pitch with the seasons and may eventually become untuneable, which also makes them unsellable! Also, find a reputable tuner (Mrs. King recommends Robert Fisher at MrPianoTuner.com in Crystal Lake, IL).
It costs $80-100 to tune a piano.