One of the problems I see most often among guitar players is playing on the wrong part of the finger. This often leads to other problems such as bumping neighbor strings. The solution is to play on the fingertip about 1/2 cm or so from the fingernail. Also, for some fingers the location is slightly off to the side rather than smack in the middle of the finger. For the first finger on the left hand side the string should come in contact with the finger on the left side of the finger tip (left as you look at your finger, closer to the thumb rather than the middle finger) The second and third fingers tend to be more or less in th middle. And then comes the fourth finger on the left hand (the problem-child). For the fourth finger you should play on the right hand side of the fingertip. This causes the fourth finger to point in back towards the thumb. I drew a picture that probably does a better job of showing this. Click on it to zoom in.

When playing fingerstyle it is important to move the fingers from the knuckles (primarily) rather than the inner joints. Why? Several reasons. It causes the finger to move in towards the hand instead of up and away from the guitar. Moving in towards the hand (a motion similar to that of grabbing a baseball bat) results in better tone. Also the muscles that control the knuckles have greater independence than those that control the inner joints. In other words when you play with your middle finger your index finger is less likely to go with it. So what do the inner joints do? They keep the curvature of the finger consistent. Not rigidly, but more like the shock absorbers on your car allow for a free ride.