If you do the straps up start from the bottom of your pfd and work your way up this way it should be fitting snugly on your body and then when you tighten the shoulder straps up so they sit comfortably on your shoulders and not so tight they pull it up but if you have the body straps at the right tension then it shouldnt slide up.
Personally i think palm have gone way OTT on all their straps and the ammount of foam they are putting in their pfds these days I much prefer the astrals aquavest and will get one as soon as my palm apex wears out