One of the most fundamental concepts to discount travel is flexibility. The more rigid you are in what you want to do and when you want to do it, the more you are going to pay. Simply being flexible about the dates you are willing to travel can save you a bundle in airfare costs. Even better is if you can be open to traveling in many different places. If you decide, for example, that you're willing to go anywhere in South America, as opposed to a particular place in South America, you'll be able to take advantage of cheap fares and save a fortune. Almost anyone who is a master of the art of discount travel will be extremely flexible and more interested in the idea of travel itself, as opposed to a particular vacation at a particular time. And in most cases, when you meet people who don't seem to make much money but travel all the time, this is the attitude they have.
The concept of flexibility shouldn't end when you arrive at your destination, however. The true discount traveler arrives with an extremely open itinerary that allows him or her to take advantage of many different opportunities. The true discount traveler understands that any new experience will be more or less equally worthwhile than any other, so if they're looking at two similar outings to different places, they'll take the cheaper one knowing that it will probably be just as interesting as the more expensive choice.
A discount traveler knows that every rigid spot in an itinerary will end up costing him or her. If you decide that you absolutely must go to a particular place at a particular time, you leave yourself at the mercy of travel agents and tour guides, and put yourself in a position where you have to pay whatever it costs.
This idea of being open should also extend into your eating habits and entertainment choices. In many countries certain foods will be radically cheaper than others, so why not try them? Drink works the same way: if you're traveling in Mexico, say, tequila will be ridiculously cheap - so even if it's not your preferred choice, if you're willing to drink tequila when you drink, you're going to save a bundle.
By being flexible like this you will not only save yourself a lot of money, but likely have a more culturally immersive traveling experience - one that will stay with you a lot longer then a highly organized "tour."