Some overall straps just won't stay put; this can be particularly troublesome with children's overall straps. You can make a few adjustments to keep overall straps in place so that they don't fall off of the shoulders with only basic sewing skills.
Take a pair of scissors (or a seam ripper) and cut the seam that keeps the buckle attached to the overalls. You should be able to pull the strap completely off of the adjusting buckle as well as the button buckle.
Rethread the strap through the buckles, but instead of reattaching the strap to the back of the adjustment buckle, loop it through where it was attached before.
Bring the strap around to the front of the adjustment buckle and thread it through again. The extra fabric will keep the buckle from sliding.
Sew down the fabric to keep it from slipping out of place, either by hand or with a sewing machine. Make sure to use matching sewing thread.
You can take a safety pin and pin the adjustment buckle in place on the back of the strap to hold it in place as well.
Make sure you study the buckle and strap before you remove it so that you remember how to re-thread it.