Just did the CIEH Level 2 Food Safety course, what you need to do is interrupt as far as possible the ideal breeding environment for bacteria, a temperature 8-68C, a damp environment, something to eat, and time. A high-sugar/high-acid environment also impedes the growth of bugs, which in this instance means if you add lemon juice, you stretch the life, and if you keep it in the fridge, preferably just above freezing (5C), you get it sussed.
With the exception of clostridium botulinum (which has no hassles with anaerobic atmosphere) vacuum packing the potato salad helps, too.
Commercial mayo is stuffed so full of other retardants it begins to have ridiculous shelf life, but who the hell wants to make potato salad so bad it needs that kind of life? If your recipe's any good, it's get wolfed inside of an hour...