Recording in stereo with two microphones is the easiest technique. However, if you only have one microphone, placing it above the strings of the piano about 2-3 feet above middle C will give you the most balanced sound.
You'll want to choose a matched pair of condenser microphones if you have them; otherwise, any two microphones will work -- just be careful of placement. Condenser microphones have the wide frequency response, low self-noise, and sensitivity that capturing the nuances of a piano require.
Let's look at the technique for stereo recording using two microphones.
Open the top of the piano to expose the strings, and place one microphone about 12 inches above the strings facing slightly inward around the second G below middle C, then take the second microphone, same distance up, above the second G above middle C, same technique as the other one. pan the microphones hard right and left.
This should give you an acceptable sound; remember, every piano and every microphone is different, so take these tips and experiment. You might find something that works perfectly for you but might not work out so great for someone else.
If you've got an extended budget, there's lots of great options. One of my favorites is to use a Shure Beta 91 microphone inside the piano, with a stereo pair of microphones outside to capture the ambient sound of the piano. This allows you to fine-tune your recording to give you more of what you want, and less of what you don't.
You may also find that, despite your best efforts, you don't care for the sound from your piano; bad tuning and bad room acoustics are among the two causes of bad acoustic piano tracks. In this case, you might consider using a synth piano -- it'll cut through the mix better, and work "in tune" with other instruments far better.