Greg is clearly the expert but I would add that when you give your leg push to get it off your rack position the bar will rise without any need to grip it and you can then grip it more firmly as it goes up.
My problem is that to rack it I've developed the habit of having the pinkies off the bar. And I find it psychologically impossible to lift the weight overhead with pinkies off. That means I have to rerack it by heaving it up and down.
Any suggestions on this? Apart from get my pinkies on the bar!