As Greg says, never think about changing the ball position on putts...keep the stance, the grip and the ball position the same as you would for any other putt.

I've found that for good lag putting the key is to completely ignore the hole. Firstly you need to determine the line and the pace of the putt. There are many ways to do this but personally I find that squatting low a good distance behind the ball and picturing it rolling along the green helps. My key thought then is to find a point on the line I want the putt to initially start on that is equivalent to a straight putt on an even pace green. This target point can be a change in grass colour, an old hole, a brown patch but something physical that rather than imagined. What I've then done is instead of thinking about a tricky lag putt, I've reduced it a straight, flat putt to a virtual target.

I then line up to the target and simply don't look at the hole. I can hit the putt with confidence as I know the line is straight and I know the strength I need to hit it with.