Like almost anybody, I'd learned how to draw perspective in school. We stopped at one-point perspective as I recall, and in the drawing I've done I've never needed more than that - perhaps two-point perspective might come in useful some day.
One thing you don't seem to learn in school when you learn about perspective, is how to place the heights of different objects with regard to each other. You know that things get smaller as they approach the horizon, and you can use the guidelines like artman2003 describes above. But say you're drawing the road in the desert like he describes, and you want to add a cactus - how big should it be? If you draw a cactus somewhere in the desert, there's no way to know if it's either a small one, standing rather close, or a big one standing far away. Or even worse, you draw a camel (it's now a drawing of Morocco in stead of Arizona), and everyone knows how large a camel is supposed to be, and somehow it always seems the wrong size or position in your drawing. I remember trying this stuff and it never seeming quite right, it never came out quite like I wanted it to.
Then at an art class I learned a neat trick. It's a simple rule: the horizon is at your eye level. Anything that is below your eye level, you should draw below the horizon. Anything that is above your eye level, you should draw above the horizon.
I'm not even going to try to put this into ascii. Try doing it yourself with pen and paper, as follows: make a simple drawing of a road going off into the distance, toward the vanishing point. Artman2003 has given a perfect example. Now draw a person somewhere on the road (it can just be a stick figure if you can't draw that well, it doesn't matter). Pretend the person is the same height as you. This means the eyes of the person should be at the level of the horizon, and a small bit of the head sticks out above the horizon.
Got that? Now draw a larger person, somewhere in your drawing, again with the eyes at horizon level. See? If the difference in size is large enough, that second person now clearly seems to be standing closer than the first.
Now draw a third person, with his feet at the same height as the first, but his head sticking out above the horizon. You can now see that although this last person is drawn bigger than the first, he doesn't appear to be closer. It's clear that he is actally taller.
With the rule in mind, it gets easier to make things seem the right size. The distance (on your paper) to the horizon indicates how far away something is: the closer to the horizon, the farther away. You can estimate how far below or above the horizon the item should go by comparing it with yourself. Say your eye level is at 1.50 meter (mine is). Then when you draw a three meter tall object, half of it should be below the horizon and half above it. If the object is only 0.75 meter tall, then it only reaches half of the distance between it base and the horizon. And so on...
This system gets a bit more complicated if you draw from a different level than normal, standing on the ground eye level. If your point of view is high in the sky, then everything is below your eye level and thus below the horizon. Or vice versa, if your point of view is just above the ground, then all parts of a person except for the feet stick out above the horizon. (Try it! Take your drawing and draw a new horizon, above or below the previous one, and add a new road for reference. You should be able to see how much difference that makes for how big your people seem). The rule that the horizon is at eye level still counts in these case, but the calculation of how far things should stick out above the horizon (or stay below it) is a bit harder.
Now that you know the trick, you can always make your cacti appear the right size. Happy drawing!