IQ is measuring something real and important to life outcomes (the so-called g factor) but it is not everything that matters to life outcomes or cognition. As Keith Stanovich pointed out in What Intelligence Tests Miss, IQ is not the same thing as rationality. Your intelligence can defeat itself if misapplied. And due to human nature, it will. The more clever you are, the more ways you can deceive yourself. Reading the Sequences (or RAZ) can help you learn to stop doing that.
Why wouldn't you want to know your own IQ? Are you afraid a "bad" result would become a self-fulfilling prophecy? If you want to be more rational, then the truth is not something to be afraid of! You have been living your whole life with whatever IQ you have. Knowing your own strengths and weaknesses doesn't change what they are. It just gets rid of the weakness of not knowing that.
There are multiple components to IQ tests. You may be stronger in some areas than others. In my case, the IQ test revealed that I have a learning disability, despite having the overall genius IQ typical of LessWrong readers (or at least LessWrong survey takers).
Learning about this was not a disappointment. It was a relief. I had always felt like I somehow wasn't measuring up to my own apparent potential, but now I realize it wasn't my fault. Now that I know what my weakness is, I can better compensate for it, and better leverage what strengths I have.
While we don't know good ways to improve fluid intelligence by much (besides avoiding those things that make it worse, like sleep deprivation, etc.) there are well-known ways of increasing your crystallized intelligence: Read more and better books. Listen to audiobooks during your commute. Use SRS and mnemonics. Specialize. You can generally out-learn someone a bit smarter than you if you develop better study habits. And you can expect to far out-learn someone who isn't even studying your field.
You can also increase your effective fluid intelligence in many useful situations by using external tools. Working memory is consistently one of the worst bottlenecks in human cognition. Write things down when thinking. Draw diagrams. Take pictures.
Learn to use a computer more effectively. Try org-mode or FreeMind or TiddlyWiki. Learn to use a spreadsheet. Try AutoHotKey to improve your efficiency. Learn Python, if you can. Try working through a math book with Mathematica instead of pencil-and-paper. There's a night-and-day difference in effectiveness between an illiterate genius and a merely bright person who has access to a PC and Google and knows how to use them.