I put the shifter into the H position and find sometimes as soon as I hit the gas it is not in gear all the way and changes to the L position... do you have this problem?
In my opinion, Polaris could have done a better job with the shift lever. I have 2600 miles on my 2010 RZR that I've owned since November 2010, and I still miss a shift sometimes.
I only have 60 miles on the ACE, and I've been struggling with the shifter too. It's been getting better - mostly I have trouble with R, and I'm getting pretty good with H and L.
You just have to get used to what the shifter FEELS like, going into those gears. Use the indicator on the dash as a guideline, but you will do yourself a disservice if you rely only on that. So watch the dash, but also pay attention to what it FEELS like as it moves from one position to another. You'll get it dialed in.
Also, as H is the last position in line, pull that shifter back with confidence. Assert yourself. If you use a fair bit of force and it's still popping out, your shifter might be out of adjustment. You can find the linkage and adjust it yourself, or I'm sure the dealer would be happy to do it for you.