You might think that there's no point recording a real piano now that there are so many fabulous ROMplers on the market, but as stunning as some of these are, there's still a special quality to a real piano.
It's the sum of countless little parts, like the sympathetic vibrations of adjacent strings when one or more are struck, all of which affect the tonal quality of the instrument. And it's not just all about getting the best grand sound either - if you want that authentic Chas & Dave pub-piano sound, you're going to need a suitably old and ropey upright, and there aren't many ROMplers of those!
We should warn you though - the piano is the single hardest instrument to mic up in the known universe. Whatever the type, all those different keys produce such a range of frequencies that it's almost impossible to find a mic position that's perfect for all of them at the same time. Not only that, but because the geographical placement of each is different, the different strings will reflect differently off the different surfaces, to say nothing of the way the different frequencies will respond to even the same surface.
The right room
Because pianos tend to broadcast a big sound all over the place, they fall prey to the same acoustic problems as studio monitors. Pianos in small rooms will cause a distinct resonating character with certain notes - and room resonance is the sworn enemy of good recording. Ideally you want the largest room you can get your hands on, but in case your ballroom is currently being decorated, you can also try moving the piano around the room - away from walls and so on - until the resonance is reduced.
If your brass section has finished monopolising your Neumman mic, get it back off them and unpack its twin brother, because you'll need at least two quality condensers...
There are more 'pro' techniques for recording a grand piano than there are keys on one. With that in mind, we're going to give you a good starting off point and some tips on customisation.
Start by placing two mics about four metres back from the keys, and about two metres off the floor. They should be lined up at an angle, roughly parallel to the angle of the right side of the piano (in other words, far left to near right, from the player's perspective). The best sound can often be found with even small movements (a few centimetres will often do it), so go slowly, and if your sound's lacking sheen and warmth then start by moving the mics back a little further away from the keyboard end.
Next up are some ambience mics, if you're so inclined. These are especially useful if you're working in a large hall or auditorium. Basically you want to handle them as you would any other ambience mic, but since the space available is often much greater with piano recordings, it's easier to just set up as many mics as you have spare, and then pick the best ones later (or even use two of them panned wide for stereo ambience).
If grand pianos can be a pain, uprights are even worse, as their closed nature makes for some hectic resonances. With this in mind, remove the backboard of the piano and set up a stereo pair (set left and right and slightly above, pointing down) at your chosen distance.
As a general rule, pop piano parts are recorded much closer - try about 25cm from the strings, but don't go any closer or you won't pick up the different notes equally.