The trick is to file the underside of the nail to create a flat plane. Stop when the file contacts the entire nail at once. Then file the nail straight on to eliminate the bevel you made. Follow with whatever very fine sandpaper you normally use.
Viewed from the top the a-nail in the picture above (the only one that you can see from the correct angle; the others should be viewed in the same way) will be shorter on the pinky side, but you can't just copy its new shape by filing straight on: you must always file the underside flat first. So this isn't a permanent fix - you can't "train" the nail to grow without a hook. I've tried to show the angle of the file by adding a little line. Remember, file the underside first to create a flat plane, then file straight on to remove the bevel. Also, look at each nail in the same angel we see the a-nail in the photo.
One nice thing about it is that, at least in the example above (in fact in most cases), you don't have to shorten the nail where it contacts the string, which means that you don't have to wait for the nails to grow or get used to the new shape - they normally feel natural and it's easy to play immediately. Furthermore, it works with all kinds of nails because it's based on the principle of filing a flat plane on the underside - where the string makes contact.
You might have to practice a bit before you ge it right but believe me, it works. I've filed thousands of nails like this in the last 30 years or so.
Here are a few more tips:
File the nails to make them grow. File them every few days to keep them the same length. Sand/polish them every day - they should always be smooth as glass when you play.
Don't apply pressure with the file (this changes the shape if you've got thin or very flexible nails), use it lightly and let it do the work.
Don't file the nails when they're wet (after a shower, for instance). They're softer and a different shape when they're wet.
Likewise, if you insist on using polish or lacquer (I have to say that I don't think this it's good for the nails to be deprived of oxygen), it should be applied and completely dry before you file the nails because lacquer can alter their shape, especially if they're thin. You want to file the nails you're going to play with, some nails of a different shape.
Torn nails can be repaired with super glue, which can also be used to glue crescent moon-shaped cuttings made from ping pong balls to the underside of a broken nail (this is more or less what a product called Players' Nails was).
Exotic and eccentric remedies should be avoided. Creams, salves, potions and unguents are, in my experience, of questionable efficacy and often expensive. It's more important to have a healthy diet, keep your hands clean and your nails smooth.
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