Ways to learn to season food correctly?

My best recommendation is to taste as you go. Taste the initial product...raw vegetable, ingredient from the can, bottle, etc. and then continue to taste and sample a dish throughout the cooking process to see how flavors develop/diminish and enhance one another through the cooking process.

Learning to season food is a process of educating your palate and developing a "flavor memory"

One of the most important factors is to use enough salt. Food that is properly seasoned with salt shouldn't taste salty but will have a brighter more vibrant flavor of the ingredients that are in the dish. Food must be cooked with salt for this to occur. Food that is seasoned at the table will merely taste salty as the salt doesn't have a chance to dissolve and pull the juices out and help them mingle with one another as happens during cooking.

The more of the basic flavor profiles that you can incorporate the more lively and flavorful anything will be. Even before cooking you can take a look at a recipe and "disect" its flavor profile by determining which ingredients will add sweetness, sourness, etc. If you notice that it's heavy in one direction or another, the flavor profile of the item(s) missing will likely improve the dish. Then it's a matter of deciding what ingredient with that flavor profile would be best to add to that particular dish.

Shameless (but applicable to the question) plug: If you're ever in Savannah, GA I offer a class called "Flavor Dynamics". It is focused entirely around understanding how flavor develops, what affects our perception of flavor, and how to create well rounded flavor in your food.

Definitely taste as you go and season gradually and regularly. Good chefs might taste their dish thirty times before it gets to the plate. Proper seasoning is not a formula, it's heat-seeking missile that constantly adjusts to hit its target. Or an impressionist painter who builds a base of color and then dabs on highlights and shadow to bring out the bigger picture.

Details: Salt is not a flavor, just a flavor enhancer. Add dried spices early, fresh herbs late. The more seasoning cooks, the deeper it's flavor, the less seasoning cooks the sharper its flavor. If you season early and late, you get both effects.

Anatomy is the base of flavor. We taste sweet, salty, acid, bitter, and umami. Also heat. If your food tastes bland, one of these is missing. If your food tastes off, one of these is out of balance with another.

Sweetness is sugar, honey, carmelized onions. Acid is lemons, lime, vinegar. Bitter is dark greens, brussel sprouts, rind. Umami is sauteed mushrooms, soy sauce, melted cheese. Other foods float between categories depending on the variety and how they are prepared.

Generally, the more you cook something, the more sweet, less acidic, and more umami it becomes. Up to a point. Overcook and food becomes bitter, bland, compost.

The rest is smell. Your nose carries subtleties of taste your mouth can't detect. Warm food often tastes better because the aroma is unleashed (also the sugars). To master flavor, smell your food. Smell your spices. Smell your ingredients. Flavor is the interface between food and human. The only tool you have is your senses.

There are almost two separate schools of thought on this. One is using seasonings and sauces to enhance and balance the flavor of the original ingredients, but that the original ingredients are still the primary flavor of the dish. The other is using seasonings and sauces, to create an end result where the original ingredients are no more than a few notes in the symphony.

In my opinion, correctly seasoned simply means that it tastes good to the people eating the dish. The hard part is that it seems to vary a bit by person. What tastes properly spicy to one person is too hot for another. Same thing with sweet, sour, and bitter. This matters most when you are creating dishes that push the envelope in any direction (hot salsas, sweet sodas, strong lemonade, dark chocolate).

@Ben: After you scoop up some of the food with a spoon, have a clean place on the counter to set it down for a minute before tasting. Don't stir with the spoon before tasting, because the longer the spoon is in there, the hotter it will be.

I think the two answers above are very good, but this is my technique. Start with salt and pepper (sea salt, course and freshly ground black or mixed pepper). Use these sparingly, until you get the hang of it. Once you find yourself saying, "geez, I wish the food had "_" flavor," look for the spice/ingredient/seasoning you need. Learn it and stick with that one flavor until you have the hang of it. Then repeat this process until your repertoire is built and a nice size.

The biggest mistake I have seen with flavors is people try to use too much and they end up ruining food because they don't know how to use the spices/seasoning. I still use way less than most skilled cooks I know and my food tastes great and is full of flavor. I think I have adopted the idea of mastering fewer rather than knowing more less... Trying to use a whole bunch of seasonings all at once, I have found, is much more difficult than taking one flavor at a time and learning it well.

If you want a less subjective answer (and you're an anal, scientific, overly careful cook like I am), you can do a pretty good job seasoning your food simply by measuring its weight. From the book Ideas in Food:

Interestingly, as we have become more diligent about recording our recipes, we have noticed that our personal salt concentrations are very stable. Across the board, regardless of the recipe, we tend to season our food at a level of 0.5 percent of the weight of what we are cooking. There are a few exceptions where the level creeps up to 0.75 percent or down to 0.4 percent, but generally speaking, our palates are amazingly consistent.
_{[Emphasis is mine.]}