first things first.
Pronation doesn't happen conciously.
You don't conciously think, I'm going to twist my wrist/hand/forearm into the ball.
Instead, try and hit the ball square in the sweet spot of your racquet, aim to hit it 'flat'.
At the same time, (and I'm going to give full credit to Tennisplayer.net here, because this is the best, [and only], way of teaching this that I have yet heard), visualise the racquet head moving from down to up, applying spin.
Pronating just the forearm will
1)Hold your serve back dramatically, it will suck relative to your potential serve.
2)Hurt you at some point, could be immediately, could be in a couple of years...
Had to clear this up, This instruction has been getting more and more widespread.
I know this is an abrupt post, I'm in a hurry, all the info is correct though.