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It is inevitable, if you are going on vacation to a place that you have never been before then you are more than likely going to look like a tourist. You can do a few things to tone down the effect though. Try to dress like the locals dress. Or at least do not wear any outlandish items that draw attention to yourself.
One thing that makes a tourist look like a tourist is the camera around the neck. For vacationers this is part of the vacation. You need to document you trip and keep record of how much fun that you had. You do not however need to take you most expensive camera on your vacation.

You may be tempted because of the great pictures that it can take but if you are afraid of the loss of that camera then leaving it home might be the best idea.
Tourists tend to be noticed almost as much as you are noticing the new things around you on your vacation. Many people can blend in but unfortunately when you are on vacation it is much harder.

The best way not to look like a tourist is to not do anything different than you would in your own home town. If you cannot help but look and act like a tourist then at least take steps to protect yourself from people that prey on them. Read up on how to protect yourself while traveling.