I imagine this depends on what the food is. With spaghetti, you can add water or change water if you detect it on time. But what about grilled meat? Or a tomato sauce?
Parsley, or other very leafy stuff. Specifically, I've noticed that adding parsley to a dish that's too salty works wonders. It's good for almost any sauce you're making, as well as pasta or casserole. Probably won't work so well with over-salted meat though. Perhaps a parsley and mint sauce for the meat?
I made a potato lentil soup and used some ham base..... Wow... Too salty.... Added a couple of packets of spllenda and two TAB of balsamic vinegar.... Really helped... Slightly worried about it as it gets used up... Wondering g if the salty flavor will return as some evaps....!???? Thinking about adding more potato to help that??
You can't remove salt from a dish once it's added, so Shog9 has the only correct answer to this question: add more ingredients to dilute the salt.
The commonly suggested "remedy" of adding potato in various forms is only really attempting to mask the salt flavor. It does nothing to actually remove the salt from the dish.
Any starch will lessen your perception of saltiness, hence why so many starchy dishes (think baked potato) require higher levels of salt to taste good.
Adding starch to the dish (potato starch, cornstarch, etc.) or serving the dish with a starch (bread, pasta, potato, etc.) can lower the perceived saltiness of the dish.
Do keep in mind that starch is not a panacea, as parts of the dish may still taste salty. Also, while starch does mask our perception of saltiness, it also masks other flavors as well, so your dish will taste blander overall.
Like a chef-instructor told us in one of our first classes: heavy cream fixes everything. Specifically, fat fixes oversalting if the dish can handle more fat. Fat coats the tongue which prevents as much salt from hitting your taste buds. If you have a very heavy, fatty sauce, it will need much more salt than, say, a tomato sauce. This makes heavy sauces challenging to season, but it can save your ass if you oversalt something.
I add a large, peeled potato or several peeled carrots to the soup or stock as it cools. Cooking the desalinating root vegetable interferes with the osmotic process. As the salty soup cools, the starchy root vegetable(s) absorb the excess salt. This is not a quick fix and requires several hours or overnight refrigeration. Remove the root vegetable(s) and reheat. PS: Peanut butter (one big tablespoon) is a wonder for over salted, burned, or otherwise "ruined" dishes. Bon appetit!