For potato salad: I have several secret ingredients. I don't necessarily use them all at the same time, but hard boiled eggs, yellow American mustard, and relish made from home-made sweet pickles.

On the topic of pickles: This is particularly true for sweet pickled veggies (like cukes & peppers), I'll add some fresh cut ginger. Pairs extremely well with more the more traditional pickling spices used in my part of the world.

For most anything savory: A bit of celery seed. This one falls into the "with great power comes great responsibility" category, though. Use just enough that you, the cook, can detect it but not so much that other people can easily pick it up in the flavor profile. Also, it works best with dishes that pair sweet and salty profile with herbs, such as soups and marinades.

For dishes that need a bit of extra fat: I'll sometimes add some bacon grease if I know that everybody having the dish is fine with pork products. It's a quick way to add saltiness, meaty/umami, and smoke flavoring to things like various greens, various bean dishes, and stuff like that.