I wear a 9, 9.5 or 10 in most shoes...my feet are on the wider side and I can't stand to have my toes in too tight a space.
I tried on the 9's and my feet slipped in easier than I expected but there still was some resistance. They felt quite tight across the top of my feet so I didn't even try an 8.5.
The gal at the store said she wears a 9-9.5 but had to size down to an 8.5 in that boot...but her feet aren't wide like mine. I'm also heavier, so my feet are meatier I guess. LOL
Anyway...I was told they'd slip about a fingers' height for a while but that they'd stop slipping. The slip a lot more now and other than wearing thick socks (I like thin ones...and it's over 100 degrees here) I am not sure what to do.