This is a problem I've had to deal with because of a narrow foot--less so with monks, where some tightening is possible (and you might think about having another hole added to the strap) than with loafers, where it has been a real problem. Mafoofan is right about new shoes, and I always make it a practice to take new shoes firmly in hand and flex them a lot before wearing them. I try to get as much bend in the sole as possible and repeat this a number of times. The other place where flexing helps is the heel, where, without actually breaking it down, I flex it a number of times too, since once it becomes more malleable, it will flex more with your foot and rub less. Then wearing them, where flexing will occur naturally, will help some.
Sometimes, however, it is necessary to go a little beyond this. Two remedies that have been suggested are: (a) heel grips (thin textured pads that attach to the inside of the heel) and (b) tongue pads that stick to the tongue, and which, at least in theory, help to push your foot back into the heel, thus reducing slippage. In my own case, I haven't had much luck with tongue pads (which have produced a little discomfort to the top of my feet), but the heel grips have helped a little. Wearing thicker socks with shoes that slip should help a little too.
I've heard that shoes can be "tightened" a little by a skilled cobbler, although I haven't had this done. Perhaps another forumer can comment on this. In the end, however, if none of these remedies solves the problem, you are right and truly screwed, and the shoes will likely just end up unworn.