Preventing straps from sliding from the victim's shoulders isn't that easy. There are a few things that you can do though:

	You can twist the straps around each other a few times, just above the splitting in the back, before fastening them. I actually got this idea by seeing someone wearing her overalls like that, though I don't know if she did it for this reason, or just was careless when putting them on. See also Ariel in Scenario: Committed, gallery 1.
	You can put a chain around the neck of the victim that includes the straps. Not so elegant, but effective nonetheless.
	Add a button in the middle of the bib, and connect the straps to that. In the illustration the button is at the back of the bib, to make a lock less conspicuous. You can also just move the buttons closer together. See for a similar example Ariel in Bright Blue Denim Overalls
	You can try to find a pair of overalls that are constructed differently, e.g. with the straps connected to the side of the bib. (There is a risk here though that the victim can pull the straps over the head.) See these also in Ariel in Purple Velvet Overalls.
	Another useful construction is when the straps buckle in the back, like with these leather-look overalls from ASOS. Originally you wouldn't need to undo the buckles, but with a little tie-wrap the buckles are in the small of the back and very hard to (un)do even without a lock. See these also in Ariel in Black Pleather Overalls.
Picture edited to show the idea	You can also add a crossbar to the straps, at the back, just below the neck.
	An anonymous fan came with this elegant solution: wear a t-shirt under the overalls, but put the straps in the back under the t-shirt, while the bib in front is outside. Then the neckline of the t-shirt prevents the straps from being pulled off the shoulders. In effect the t-shirt prevents the overalls from being taken off, and the overalls prevent the t-shirt from being taken off! (Of course you do still need to lock the buckles on the overalls.) To demonstrate this I've made a mini photo shoot with an inflatable mannequin I call Argenta.
	Other options include: use a shirt that has loops at the shoulder (orig. for a baret), and thread the straps under those. Make sure the loops can't be opened though, e.g. by sewing close the buttonholes. See for instance Ariel in Blue Corduroy Overalls