That's an excellent question that comes up often.
You have to decide whether you want to place it for convenience, for security, or for performance.
For convenience, you might want it somewhere easily reachable by your right hand, such as on your left forearm or left upper arm. You also want it somewhere that is easily visible and readable by your eye, but that varies from user to user. A few men's shirts have a small pocket on the outside of the upper sleeve, and I've put them there before. You can put it in a cargo pocket on pants, but there is a greater chance of signal blockage by your body there.
For security, you would want it in some kind of holster, maybe on the outside of your hipbelt, or else inside something in a shirt pocket, if you have a shirt pocket. Note that just about all fabrics are transparent to the GPS signal unless they get soaking wet, so don't worry too much about covering it up. Some users put the actual unit on an 18" tether that is fastened to the inside of the holster. That keeps you from forgetting and leaving it on a rock. However, there are times when you really want to set it out on a rock for a few minutes while you stroll around.
For performance, you need to know exactly where the internal antenna is under the case, and you want that facing as much upward as possible. A good place for that is underneath the top flap on a backpack. However, that is difficult for the convenience aspect. At a minimum, you want it pointed outward, but upward is far better. If you leave the antenna oriented poorly, it will still work, but you will get increased lock times and increased position error. If you get under a tree just wrong, you may have a loss of signal.