So, what it boils down to is: you have to find the string before you can play it. Preparation is the conscious practice of "finding the string". The topic seems to be somewhat controversial, or at least some guitarists say they never have practiced preparation and wouldn't touch it with a ten foot pole, but I think the priciples are valid and, if practiced the right way, can be very valuable.
What's the simplest application of preparation? Place your 'i' finger on a string. Pause a few seconds to be sure that your finger is "seated" properly, i.e. the spot on your fingertip/nail that you want to be in contact with the string is in contact. Relax any excess tension. Play the note. Viola. You have just practiced preparation. O.K., what's the next step? Let's alternate 'i' and 'm':
This kind of preparation, where only one finger at a time touches the strings, can be called "sequential" preparation. I find it useful to practice this even with one finger at a time, to develop an optimal point of contact on the finger; that is, play a string with the same finger several times, pausing each time the finger is returned to the string to check placement and relax any excess tension. (This last point cannot be over-emphasized: if you develop tension in your hand while practicing this way you will defeat yourself.)
Practicing the Villa-Lobos Etude #1 pattern with sequential preparation is one of the most useful arpeggio exercises that you could do.
As testimonial to the effectiveness of sequential preperation in rest-stroke 'im' scales -listen to flamenco guitarists. You can often hear them using the preparation (which makes the scale sound stacatto) as an expressive device, and also at the beginning of a long scale, before an accelerando. You can tell that they practice this way all the time.
Now we get to the touchy point: simultaneous preparation. For playing two or more notes simultaneously, there is no problem: place the two or more fingers on their respective strings, check the points of contact, relax, and play. But what if you wanted to play this most basic arpeggio pattern?:
-----------a---One can, does, and must be able to play this sequentially - only one finger at a time contacts the string. But what about placing all four fingers on their respective strings, as if you were going to play a chord, and then sounding them one by one, leaving the fingers on the strings until they play? some people feel this is not good - both because of the muscular motion it trains and for the way it effects the sound.
Especially if you are playing this pattern repeatedly, as in Villa-Lobos Prelude #4, each time you place the fingers down you are stopping the vibration of the strings. This is like lifting your foot off the sustain pedal of a piano at the beginning of each beat. Now, if you did this all the time because you could do nothing else, it would be a limitation, and therefore undesirable. However, if you *want* this effect, then it is an enhancement of your control of sound, and therefore desirable. Also, I can vouch from personal experience that I can play this pattern much faster when I place the four fingers together, than when I play it sequentially. (Actually, what I think I am doing is placing the thumb first, then placing the three fingers together as the thumb note is being played) Again, the danger to the muscle memory would be if you *always* played this pattern with simultaneous preparation, it would limit your abilities in similar patterns where you did not want to prepare the fingers. That is why it is important to practice a pattern like this both ways.