Seasoning often seems to boil down to a simple question: How much salt to use? Most chefs will tell you the answer is probably more than you're currently using, but not as much as you'll find in your typical can of chicken broth. Salt is the mother of all seasonings, with pepper a close second, so getting the right amount is a crucial skill.
It's a balancing act, and the only way to learn is to taste everything you make, at every step, all the time. (It may also help to imagine an impatient chef standing over your shoulder saying "needs salt," which in my experience is the one thing an professional chef would be most likely to say if he or she were in fact standing over your shoulder.)
But there's another aspect to seasoning, namely how do you get the right amount of seasoning onto the right piece of food just in the nick time, when you're also in the middle of doing a zillion of other things? Whether it's a screaming chef on the hot line or a screaming toddler clinging to your knee, most cooking in the real world is done under duress, so practicalities matter. Put down the salt shaker, drop the measuring spoons, and learn to season with your fingers.
First off, have salt and pepper ready in little tins or bowls, right next to your stove or wherever else you are likely to need them in the heat of battle. Otherwise you'll forget to add them at the crucial moment. Keep them there all the time, refilling them every few days as needed.
Next, think uniform distribution. If you've got a tray of salmon fillets to grill, they should all get equal treatment so one isn't saltier than the next. This is easier to achieve if you aim high, holding your fingers well above the food so the grains have a chance to spread as they fall. Some chefs drizzle; others use more of a toss, and more than a few add their own dramatic flourish.
This is the quickest way to season, and -- just as important -- it looks cool. Gracefully arcing a stream of white grains through the air into a hot saute pan is just much flashier than using a shaker. (Or so it would seem -- all those guys on the Food Channel can't be wrong.)
Learn to pick up just the amount you need each time, so you don't have to double-dip. And if you're really good, learn to keep one hand dry while cooking so you can do this without having the salt gunk up on your fingers.
Finally, use kosher salt, which doesn't have iodine. Sea salt is also great, but it's expensive, so I'd save it for special occasions. Some cooks say the coarser grains in kosher salt help it to cling better to the surface of foods, drawing out the moisture, and aiding development of a nice crust when searing. True or not, the larger grains are easier to sift between your fingers, and the texture just feels better.