Cutting on salt intake but never want to sacrifice food flavor? Well, a good salt replacement is MSG. As in use MSG instead of salt. The flavor loss by removing salt can be covered by the mentioned substance.
There is more explanation to this than what your senses could perceived. According to wiki, the taste of low-salt foods improves with MSG even with a 30% salt reduction. The sodium content of MSG is roughly a third of the amount (12%) than in sodium chloride (39%). A nice sodium reduction figure. Thanks to good interaction of MSG and salt.
Menshealth.com also pointed out that salt is indeed a better salt alternative. It helps keep your salt intake low and makes your food tastier. MSG consist of sodium and glutamic acid. The latter is a naturally according substance in variety of agricultural produce. It explains why some foods in its original or slightly modified state are naturally flavorful. The case of mushroom.
After ditching out the salt and using MSG for sometime, you realized that you also need to eliminate the latter. Someone from Yahoo Answers said that a fair substitute for MSG is a 50/50 mix of salt & sugar with a dash of fish sauce per teaspoon of the mixture.
Several friend of mine and my very own wife also uses dash of sugar as replacement for MSG. I think they are adding salt to taste plus a dash of sugar.
Ask.com recipe labs recommends to use of fish sauce as the best direct substitute. Worcestershire sauce or soy sauce will also do.
A good replacement of salt is MSG and the replacement for the latter is the first. Maybe it is better not to use both.
Fitday.com suggest other alternatives like, aged cheeses, meats, seaweeds, mushrooms, tomatoes, red wine, asparagus, anchovies, sourdough bread and walnuts. If one of these is naturally part of the recipe, then it is fine. Subject to experimentation if not.
I personally use onion to replace both salt and MSG. Always adding twice number of heads whenever the recipe needs it. Onion is almost always available.