As an European, I did never have any IQ test, nor I know anybody who (to my knowledge) was ever administered an IQ test. I looked at some fac-simile IQ tests on the internet, expecially Raven's matrices.
When I began to read online blogs from the United States, I started to see references to the concept of IQ. I am very confused by the fact that the IQ score seems to be treated as a stable, intrinsic charachteristic of an individual (like the height or the visual acuity).
When you costantly practice some task, you usually become better at that task. I imagine that there exists a finite number of ideas required to solve Raven matrices: even when someone invents new Raven matrices for making new IQ tests, he will do so by remixing the ideas used for previous Raven matrices, because -as Cardano said- "there is practically no new idea which one may bring forward".
The IQ score is the result of an exam, much like school grades. But it is generally understood that school grades are influenced by how much effort you put in the preparation for the exam, by how much your family cares for your grades, and so on. I expect school grades to be fairly correlated to income, or to other mesures of "success".
In a hypothetical society in which all children had to learn chess, and being bad at chess was regarded as a shame, I guess that the ELO chess ratings of 17 year olds would be highly correlated with later achievements. Are IQ tests the only exception to the rule that your grade in an exam is influenced by how much you prepare for that exam? Is there a sense in which IQ is a more "intrinsic" quantity than, for example, the AP exam score, or the ELO chess rating?
Short answer: it's not preparation. Sure, if you study the answer key of a test, you'll get a better score on that test. However, there's no known method (including practice) that increases the cognitive ability (Spearman's g factor) that IQ tests measure. Some IQ tests have no behavioral component at all; they just scan your brain and calculate your IQ.
For a solid primer on IQ, I recommend Intelligence: Knowns and Unknowns. It's a consensus report of a task force of the American Psychological Association, so it's as credible as anything.
Personally, I took an IQ test for toddlers when I was about 3 (my neighbor was a psych grad student who wanted to practice giving the test and my mom wanted a short break from having a toddler). I got a 168 (the limit), surprising my neighbor quite a bit. I had done about as much test prep as the typical toddler (none, unless Sesame Street counts). I haven't taken an IQ test since then, but my life experience since then indicates that the test was qualitatively accurate.
Some people are naturally good at IQ tests and some people are naturally bad at them, and there's not much a person can do to change their scores (aside from brain damage, of course). The people who are good at IQ tests have an advantage in any situation where absorbing, remembering, manipulating, and applying information is useful, which is a lot of situations. The people who aren't have a disadvantage in those situations, and (with our current technology) we have no way to help them.