I have three Maggies (you can see all three in the thread I just posted) and they are the smalls, and i have ZERO issue with keeping on my shoulder. I absolutely ADORE the Maggie line (well, now I am falling in love with the Kristin but that's a whole different subject)
I just picked up the bone yesterday but I think I love them all the same (spoken like a typical mom, huh?). All three colors I purchased because I feel they are versatile. I had the bone shipped in from a store in another state.
I don't know if you can see this or not, but if you look at the Periwinkle, I have the outer strap crossed over the back strap which prevents slippage. I do that automatically after purchasing the Parker Satchel and having the issue with straps.