Very bad advice.
So what is important, Davis937?
The answer is a bit complicated, but I hope you'll forgive me my primitive English.
The most important thing is a rigid structure of your arm and body. This kind of structure is very popular, especially in martial arts. You can see it
here (at the bottom). Hand is straight, line between your palm and the centre of mass of your body is perpendicular to the line of shoulders. In tennis this structure is usually a bit different, because you have a racquet in your hand. So, the line between the centre of your racquet's head and centre of your mass should be perpendicular to the shoulders.
As you can see, biomechanics (was there any?

) gives you an answer, why do you hit volleys "in front of you". If a ball is on the left or right, you should rotate this structure - that's why many people say "turn your shoulders".
So, first advice: build a rigid mechanical structure.
Now there are two possibilities.
Volley is a unique shot in tennis, because it's is common that you change your contact point, depending on the situation. You should always go just into the ball, along the direction of its flight. That's the best situation: you have rigid structure of arms and body, ball is perfectly before you (ball, centre of racquet's head and centre of your mass are in line), you slightly move your racquet. You do not need to turn shoulders in this situation, just hit the ball and voila! - what a winner
Unfortunately, usuallly it's not possible to be exaclty on the path of flying ball and contact points are somewhere on the left or right. That's not the best situation but you can minimize probability of mistake here. How? Build as rigid structure as you can and rotate it to hit the ball on the left or right. That's why you turn your shoulders and "hit in front" (of you, in your own frame of reference!).
To make easy things easier:
- if you have time, hit the ball as soon as possible (your shoulders do not rotate in this case).
- if you do not have time and the ball is on the left or right, build as rigid structure as possible, turn it and the racquet's head "will choose" the best contact point.
Next advice: do not swing. You do not have to, your structure is rigid.
Next advice: slice agressively. If your structure is rigid, slicing is natural and gives you more control.
Next advice: if you want to slice, move the head of racquet above contact point - you have to have place to produce slicing motion. It's a bit stupid to hit the ball and slice after the contact...
Next advice: there are two kind of volleys: in place (both feet on the ground) or in motion. First ones are typical for doubles, second ones are typical for singles. If you hit a volley in the run, you should hit before your front leg touch the ground. Very important, fundamental rule! It guarantees that your centre of mass will move fluently at the moment of stroke. If your foot lands at the contact, COM shakes, if your foot lands before, COM is still not stable.
The last advice: read carefully my post, then find a match with Davydenko and watch carefully his bad volleys (in replays). Then do the same with Federer. You will have a lot of fun.