There is no substitute for salt.
Sorry.
It has effects on the tastebuds and the body that cannot be replicated.
However!
They can be approached to a degree that most people will find interchangeable.
Acids can mimic the flavour-enhancing qualities of salt in a big way.
The more acid you have in a dish, the less salt you will need.
Beware of going too far, of course, as too much acid is just unpleasant and will need salt to counterbalance.
Heat--as in capsaicin, chili peppers--can produce much the same effect.
Echoing papin's information above; NA and K (sodium and potassium) exist in balance in the body.
Too far in one direction
and you have a heart attack (too much K), too much in the other and
(if memory serves; I could be wrong)
you essentially drown (or is it edema?
I can't remember).
If avoiding sodium is related to a health concern, please speak to your primary healthcare provider before adjusting your diet in this way.
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Our taste buds are tuned for salt (NaCl), but potassium chloride (KCl) comes pretty close in reproducing the sensation.
There are a few commercial salt substitutes that incorporate KCl ( Nu-salt , Morton Salt Substitute in the US).
As sodium is an essential mineral and potassium may be hard to get out of the body, make sure you ask a doctor before completely eliminating salt.
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When my doctor told me to start cutting my salt intake, I headed to the spice aisle at the grocery store and bought every "salt-free" blend they had and started trying them.
It was one of the best things I did, because I discovered how much I'd been relying on salt for flavor and how inadequate that was.
I now buy my spices from The Spice House (much fresher than the grocery store) and really like quite a few of their salt free blends .
Most spice suppliers have a similar category available.
Buy a bunch and start substituting when you would otherwise use salt.
Some of the experiments will be fabulous, some not as much, but rarely are they actually bad.
cooking.stackexchange.com/users/104/J Wynia
I use lemon juice and vinegars to approximate saltiness in foods.
Lemon juice works particularly well in soups.
I use 1/4 cup of lemon juice as a flavor enhancer in pots of soup of eight to 12 cups.
Obviously, this is a subjective measure, and I'd recommend adding the juice by the tablespoonful, tasting the soup, and then adding more juice until you achieve the level of "saltiness" you want.
Vinegars take experimentation because they tend to be stronger and there are so many different kinds, but I like to use them with sautéed vegetables.
Malt vinegar is particularly good on roasted potatoes and french fries (I also add it to my baked potatoes before adding other toppings).
Oh, and because broth is so common in cooking as a flavoring component (and store-bought broth is so high in sodium), I recommend Herb-Ox Low Sodium Bouillon in packets because, while it's not as tasty as homemade broth, it's convenient to use when one only needs a little bit of broth.
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Most of the time you use salt for making the food taste better.
If this is the purpose, you can substitute lots of different spices and herbs.
I cook Asian-style food without the usage of any salt.
Instead I use a lot of garlic and onions.
Ginger powder is quite good enhancing other existing flavours what salt is sometimes used for.
Otherwise, it depends on the particular flavour you want to create.
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If you have high blood pressure, then you may be looking for low-sodium salt.
LoSalt is the only brand I've ever sampled and there is no obvious taste difference.
If, on the other hand, you are looking to add a little taste variety to dishes, then you might try fish sauce, soy sauce or anchovies.
Using parmigiano-reggiano or dashi will also give you the umami flavour.
All of these however have a high salt content.
Avoiding salt altogether
, you might try various herbs and spices.
Really anything will add flavour, but sage has a certain saltiness, as does vanilla for sweet dishes.
You might also try a little lemon juice or vinegar in sauces.
cooking.stackexchange.com/users/1549/Chris Steinbach
If you are used to eating a lot of salt, you may first need to accustom your palate to enjoying the natural, un-salt-enhanced flavours.
Lemon juice, spices, or other pungent/aromatic ingredients are a good way to keep your dishes flavourful without salt.
You may initially find that you're missing the "salty" taste itself.
The cure for this is just to wait for your palate to re-adjust.
cooking.stackexchange.com/users/4504/Allison
Lime juice, tamarind paste, vinegars, mango powder, sour yogarts are what I use with a little as possible of table salt.
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I try to keep my sodium intake fairly low to ensure that my blood pressure doesn't get too high.  Salt is such a common part of cooking, and it's especially difficult to cook Asian-style food without a lot of salt due to its reliance on salty sauces such as soy sauce, oyster sauce, etc.  I have tried salt substitutes such as potassium chloride but that can be particularly nasty on the palate, giving the food a chemical taste.  What are some good substitutes for salt in food?
What are some good substitutes for salt for those on low sodium diets?
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<substitutions><salt><sodium>
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cooking.stackexchange.com/users/141/LeopardSkinPillBoxHat
Our tastebuds are tuned for sodium chloride, so substitutes will often taste wrong. Potassium chloride is a common substitute, which may taste acceptable. If you are used to a lot of salt, try accustomising yourself to more natural flavours. Acids like lime juice, tamarind paste, vinegars, sour yoghurt and mango powder can also mimic the flavour-enhancing properties. While you cannot fully substitute for salt, acidic foods can be a good replacement. You can also try low-sodium salt. 
You cannot substitute salt. Try using spices instead of salt. Acidic foods can be a good replacement. Try low-sodium salt or potassium chloride. You will need to accustom your palate to eating unsalty foods.