If good cooking is about improvisation and flexibility, it's also about certain rigid rules. One of these is the culinary version of the ratchet effect: once you add certain ingredients, you can't unadd them. The only path open to you is forward. There can be no retreat. Cayenne pepper is the classic example. Once it goes in, it's in, and if you put in too much, you're stuck. Either you issue the necessary warning to the people who will be served your 10-alarm chili, or you empty the pot into the compost bin and start anew, taking special care with any high-heat elements.
Salt is similarly impossible to extract once it's gone in, and oversalted food is commonplace in the wondrous realm of prepared and packaged items. Saltiness, in fact, is often the preeminent characteristic of ready-to-eat foods plucked from the grocery shelves, just as overweening sugariness is so often the only flavor you can detect in commercially prepared desserts. Recently I gave a friend recovering from minor surgery an attractive jar of tomato-basil soup from Lucini, a purveyor of various Italian delicacies. One of the soup's virtues was that all you had to do was heat it up in a pot and eat it a simple enough procedure even for someone not feeling well. A few days later the call came: the soup was good, but too salty. I apologized on behalf of Lucini, noted what a common shortcoming this is, and then proposed an easy remedy. For saltiness, unlike chili heat, can be masked. You can't get rid of it, but you can cover it up, the same way you might paint over hideous wallpaper.
The tools are simple: sugar and acid, whether from lemon or lime juice or, in a pinch, white vinegar. Sugar and acid are most effective as a duo, since each helps balance oversaltiness from a distinct angle. But you can use just one and still succeed. Sugar is a little gentler, while acid adds a zing that distracts from saltiness at least as much as balancing it. I just add little pinches of sugar to a too-salty dish, stirring them in, until harmony is restored. Then, maybe, for good measure, a discreet dribble of lemon juice. Follow-up question: could judicious salting rescue a too-sweet dessert?