How do I avoid my hand being pushed upwards when I play rest strokes with my middle finger? Because my middle finger is (obviously) longer than my first finger, it seems impossible to avoid this. ???
If you let ur thumb rest on the E or A string (like in flamenco music), you can use ur thumb as pivot.

I'm guessing you play bass?

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Make sure your right hand is curved and relaxed. The back of your hand should be parallel to the top of the guitar, but the fingers and thumb should be curved as if holding a cup.

Start like this... hold your hand out as if holding a cup. Now, gently bring your hand down over the strings until the back of your hand is parallel to the top of the guitar.

If there is too much tension in your hand, or if your hand is too flat, you will experience the problem you are experiencing.

Conventional classical guitar technique says NOT to rest the thumb on the bass strings.

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I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.