Start by panning the piano track based on how you want it to fit into the mix. If the piano is the center of attention, it might be best to pan the piano so that its focus is in the center. If the piano is one of many instruments in a mix, it might work better to pan the piano off to the side so that it takes up less space. Alternatively, panning a stereo piano track hard-left and hard-right will leave the center open for vocals and other instruments.
Equalize the piano to change the way it blends with other tracks. This will also affect the mood it portrays. De-emphasizing the frequencies from 17khz to 20kHz will make a piano sound darker, taking away some of its air and sparkle. Alternatively, boosting the frequencies above 17kHz to 20kHz will add more brightness. If the piano sounds thin, emphasize the low end by boosting the equalizer to around 100Hz to 250Hz. If the piano has too much bass, de-emphasize the low end by cutting the equalizer to around 100Hz to 250Hz.
Compress the piano track to contain its dynamics. Compression will even out the differences in volume in a performance. A more natural-sounding track, in a jazz song for instance, will need less compression than a piano track in a rock song. Use your ear to determine the appropriate amount of compression for the piano based on the song.
Add reverb to the track if you'd like to increase the sustain of the piano. In many cases, it makes the most sense to match the length of the reverb decay to the reverb heard on other instruments in the mix. If the drums have a reverb decay of 500 milliseconds, try adding 500 milliseconds of reverb decay to the piano. Dense mixes will often require little reverb, whereas sparse mixes may allow you to add more. This is because reverb in a dense mix can muddy up the mix, since the reverb won't be able to be heard above the other instruments. On the other hand, the reverb will be much more pronounced in a sparse mix with few instruments.