While it might seem obvious when stated, it's important to remember that layering should be about combining multiple different sounds that play the same tone and in the same rhythm - don't pick layers that sound too similar! A piano is a very pure sound because it's a relatively small number of sine waves building the fundamental frequency and the harmonic overtone series (i.e. hitting middle C on the piano will play C, but also lightly produce the C above it, the G above that, the C above that, the E and G above that, etc., etc.). Other sine-wave based sounds might not stick out as much. So when thinking about your layers, consider looking for a sound that has a drastically different character, or a related sound in a different octave.
