It depends on what you mean.
You can easily make your score on an administered IQ test higher, even one given by a trained professional. If you know which test you're going to be given, you can prepare yourself to make any number come out of it that you like. Case in point, when I was in high school I had to have testing done every semester as part of my IEP. They used the same test protocol every time, so I looked it up online and read about it, and decided that as a joke I would try to make a test come out as an obnoxiously high number. In the end, I was able to practice and prepare for the test without too much effort in order to "game" it, and I was able to get a score of 196. I like to think of myself as clever, but that's not even remotely realistic (there may be a dozen or so people in the world with IQ that high).
So yes, even as an adult it is possible to increase your on-paper IQ score (or strategically decrease it, if you're looking for a prescription to whatever the hot new study drug is these days) quite dramatically and without great difficulty. It should be pointed out that 1.) people who brag about their IQ are losers, so you don't really have much to gain from doing this, 2.) skills in the tasks measured by IQ tests aren't known conclusively to generalize (while mental visualization ability is a general skill that will lead to better performance on a shape rotation task, practicing shape rotation isn't on its own known to improve general ability to mentally visualize) so "practicing" for one probably won't actually lead to any significant gains in ability and 3.) you may be cheating yourself out of a diagnosis of an underlying learning or cognitive problem, if one exists (I already knew more or less exactly what was wrong with me anyway).
But if you actually want to make yourself smarter, that's a trickier beast. It's certainly possible. Like every other organ and structure in your body, SAID (selective adaptation to imposed demands) applies to your brain, and neuroplasticity is a well-documented phenomenon. Practicing of skills can lead to changes in the structure of the brain and gains in ability that cannot be attributed to mastery of those skills alone. Improving on one's physical health for better prevention of fatigue or better energy leads to better motivation, which enhances one's ability to solve problems. Better cognitive and learning skills will effectively make one more intelligent while not altering the brain's raw processing power. And obviously all the processing power in the world can't beat out experience and rote memory when it comes to solving problems. So yes, you can become more intelligent. But what isn't known is where the line between mastery of skill and change in processing power lies. If you become really good at mental arithmetic, for instance, is that merely the result of mastery of mental arithmetic or has there been a change in the structure of your brain to facilitate mental arithmetic? It seems likely that the correct answer is "both", to a varying extent.
Intelligence is not strictly "fixed" as a personality trait when you enter adulthood, but in general it is a safe assumption that people do not change frequently over the course of a lifetime. For instance, if you know someone who is consistently crabby and temperamental now, how much reason do you have to expect that his personality will be significantly different 6 months in the future? And to return to the analogy to improving one's physical strength, if you meet a person who is of average build tomorrow and has been his entire life, how likely is it that he'll develop an athletic level of tone within the next year? It's not that it's impossible by any stretch or even especially difficult to increase your intelligence, just that it's rare since most people will not be interested in making directed efforts towards doing so anyway.
. My wife often scolds me. I can calulate special relativity, understand space expansion. But can't fix fan, refrigerator even changing bulb. 
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